Marvel Comics has some of the greatest villains in history that grace its pages every book it puts out. From Apokolips to Ultron villains give the heroes tons of trouble. This will list off the worst of the worst, the ones who cause the most destruction and have the most kills under their belt. (I was going crazy trying to figure out the order so these are in no particular order, they are just the 25 worst.)
Ultron
Contrary to MCU belief Tony Stark did not create Ultron. Hank Pym, (the original Ant Man) created Ultron while trying to advance robotics. But Ultron rebelled and turned on his creator wanting to rid the world of humans. With Adamantium and Vibranium alloy, the same material as Captain America's shield refortifying his armor, Ultron is nearly impossible to stop. And with his ability to tap into the inter-web and take over the world with a single thought, only the combined powers of the Avengers and SHEILD has ever been able to stop him and it's never an easy task even then.
Galactus
Known also by The World Eater, Galactus has one giant appetite for eating entire planets whole. Able to give out the power cosmic to his heralds--a powerful force that created the Silver Surfer--who find him his next meal, his heralds protect him using their bestowed power and usually striking a bargain with him to protect their own world from their master. And yet his Herlads like Silver Surfer are some of the few people to actually be able to stop him.
Magneto
Max Eisenhardt (no his name is not Erik Lehnsherr) was a young jew during WWII. While in the Auschwitz prison camp his mutant power manifested, giving him the ability to manipulate magnetism. One of the most powerful mutants ever, Magneto wants nothing more than have his fellow mutants destroy humankind thinking that mutants are the more worthy race. Complete opposite of his old friend Professor X who would do anything to live in harmony with humans. Father of Wanda and Pietro Maximoff, and leader of the Brotherhood of the Evil Mutants, Magneto has given the X-Men and many other heroes trouble ever since his creation in 1963 by Stand Lee and Jack Kirby.
Sentinels
From the mind of the military scientist Bolivar Trask, the Sentinels were created to do one thing. To destroy all mutants. The robots have killed thousands of mutants violent or not, and have even destroyed an entire island of mutants after a horrific battle on Genosha. The Sentinels have been used by many villains who over took a few, by magic or a rewiring of protocol, using them to destroy a city or help fight some heroes.
Kang
A time traveling conquerer born in 3000 A.D. with vast powers and technologic supremacy, Kang is one of the Avenger's most dangerous foes. Battling the Avengers on many occasions since his debut in Avengers #8. When he first time traveled he was an Egyptian Pharaoh Rama Tut, and then far into the future he is Immortus, who knows the past so well that he has in some occasions helped the Avengers survive. With his ability to time travel and knowledge of time Kang is unpredictable and will do anything to make the future a better place.
Apocalypse
One of the very first mutants ever born around 3000 B.C., En Sabah Nur was born with his blue markings on his face, and was viewed as the savior. Fighting king Rama Tut (yes Kang) to claim the right to rule, but lost too his futuristic weapon. Apocalypse grew to believe, as he found that he was a mutant, that only the strongest whether super or human are aloud to live. His armor is celestial technology. At one point he uses mutants to do his bidding, and dubs them the Four Horsemen. One of which is Angel (or Archangel) using Angel's grief over the loss of his wings to turn him, Apocalypse turns Angel's skin blue and his wings to an organic metal. Angel soon betrays Apocalypse with help from the X-Men, who are Apocalypse's biggest threats. His powers are great and no body knows the extent of his powers. He can control his body on the molecular level, giving his the strength to maybe even surpass the Hulk.
Thanos
Born on the plant Titan (fitting), Thanos is one of the Eternals a human race augmented by the Celestials to be better. After being given the Deviant Syndrome, making him susceptible to mutations, Thanos grew into his vast strength and power. With an intense need to find the Infinity Stones to put into the Infinity Gauntlet, to give him infinite power over the universe Thanos does not like to lose. After dying in the hands of Adam Warlock's Soul Gem, Thanos met Death (who just so happened to be a woman), and they fell in love. But as Thanos returned to the land of the living he realized that he would never see his love again, because he had become to strong to die.
Doctor Doom
Victor Von Doom is the king of Latveria. With diplomatic immunity it makes it very tough for our heroes to do anything to him. College classmates with Reed Richards (Mr. Fantastic), and Ben Grimm (The Thing), Victor used to be friends with the two. After an explosion of one Doom's experiments scarring his face, Doom forever held Reed responsible. Given the ability of magic through his mothers bloodline, and a highly intelligent man, Doctor Doom is not someone you want to mess with. He has lead many evil teams to try and take over the world, most notably is the Dark Cabal.
Red Skull
The founder and leader of Hydra back in WWII the Red Skull was like his arch-nemesis Captain America, a super soldier the only other. (Hitler/Nazis were just someone used to better hide Red Skull/Hyrda). Johann Shmidt was taken up by Hitler himself after he saw the anger and hatred in his eyes, so Hitler wanted to make him the perfect Nazi. Johann went on many missions for Hitler as the Red Skull (he had given Johann his Red Skull mask, which become permanent after a mishap with his own weapon The Dust of Death), doing whatever he asked no matter what and was very good at it. Soon Johann killed most high ranking generals and Nazi becoming the third most powerful in the Third Reich, and was then feared by even Hitler. Willing to do anything for the Cosmic Cube, murderer of millions, and cloned for a longer life span. The Red Skull is one of the oldest and worst villains in comics today.
Loki
Prince of the Frost Giants and son of Laufey, adopted by the Odin ruler of Asgard and the Nine Realms, Loki grew into a bitter god. He became distraught after finding out his heritage, thinking that his entire life was a lie did not sit well with the God of Mischief/Evil. Growing up in his half brother Thor's shadow his entire life, made him angry and impulsive. Wanting nothing more than to rule asgard by killing his step father Odin and to finally defeat Thor, Loki would do anything even bring on Ragnarok, the end of days.
Dark Phoenix
Jean Grey one of the original X-Men and one of the most powerful telekinetic/telepathic mutants alive, and was unpredictable even as a hero. Jean bonded with the Phoenix Force, one of the oldest cosmic entities representing life that which has not yet been born. The force took over Jean's body, duplicating her by absorbing a piece of her consciousness, in return creating a clone of her. Which was it's downfall, as Jean's emotions clouded the Force's judgement turning it into the Dark Phoenix. The Dark Phoenix devoured a star, the resulting super nova killed billions and Shi'ar Empire knew that if the universe was to survive then the Dark Phoenix, who wanted nothing more than to destroy the universe, had to die. The Dark Phoenix and Phoenix have taken over many heroes including Emma Frost, some other X-Men and even Jean's daughters/clones.
Skrulls
A green shapeshifting alien race that has the tendency to be extremely powerful, the Skrull race has given Earth more trouble than it can handle. Ever heard of the Secret Invasion? Yeah, they took over half the planet and most of the heroes (which in turn created Norman Osborn's evil 'Avengers'), most notably Spider Woman. With the ability to shift into anyone, and even take their powers (though they have to be genetically modified into a Deviant Skrull to be abel to do that), the Skrull could get into anywhere, and be on any team even the Avengers or X-Men. Let's not forget the famous Super-Skrull that was endowed with the all Fantastic Four's powers.
Kree
A race much like the Skrulls (though the two races hate each other), the Kree have been involved in a Kree-Skrull War for thousands of years. Lead by a cybernetic/organic super computer named the Supreme Intelligence. After setting on the planet Uranus, they found Homo sapiens on Earth and were fascinated with us. To help fight the Skrulls the Kree experimented on humans, but soon abandoned the project even with successful results. Those humans would later be dubbed the Inhumans. Captain Marvel (the guy) was the Kree Empire's hero, who in turn created Ms. Marvel with Kree technology turning the new woman Captain Marvel half Kree and giving her her powers.
M.O.D.O.K.
Despite how weird or funny MODOK may look, he is not easy to take down. Created by the evil science corporation A.I.M. (Advanced Idea Mechanics) to lead them to rich's he is one of the smartest beings on the planet. First he was human, George Tarleton born in Erie, Pennsylvania, then his head became too big for him too hold up after experiments on him. So they gave him his famous hover chair (dubbed the "Doomsday Chair") to keep him afloat. M.O.D.O.K. has immense psionic abilities, and has near clairvoyance. He is the undisputed Leader/CEO of A.I.M.
The Leader
Like the Bruce Banner, Samuel Sterns was affected by gamma radiation, though his accident was while working late one night as a janitor. Unlike Hulk who was given immeasurable strength and size (and a way to turn human, Sterns is always green), Sterns was given super computer like intellect and his head grew to make room for his enormous brain. The Leader will do anything to kill the Hulk, maybe capture him and do some tests on him. But he wouldn't mind turning everyone else on Earth into gamma radiated freaks too. He was also apart of the Thunderbolts.
Mister Sinister
Nathaniel Essex was born in 19th century London and was a biologist, who highly believed in Darwin's theory of evolution. It wasn't until he was taken underground after being thought of a freak, by the Mauraders that he found his place. Nathaniel Essex was apart of bringing En Sabah Nur (Apocalypse) back to life, and in return he turned Essex into an immortal alien looking mutant named Mister Sinister. Sinister is a genius and with his shape shifting, telekinetic, mind control, and a superhuman body he is not one the X-Men can take down easily. He has created the Legacy virus (the cure was dispersed thanks to Colossus giving his life) which would kill most who came into contact, he has killed thousands of mutants, and worked for Apocalypse until his apparent death and after that he was even more dangerous.
Mephisto
Marvel's version of the Devil himself, Mephisto isn't exactly a villain. He is evil incarnated, he is sin. He took Johnny Blaze's soul, creating Ghost Rider. He erased Peter Parker and Mary Jane Watson's marriage from existence. Mephisto is constantly battled for the right to rule Hell, but always seems to win, he is meant for the job it seems.
Vulcan
The third Summers brother along with Scott (Cyclops) and Alex Summers (Havok). Emperor of the Shi'ar Imperium, Vulcan has an entire army at his disposle including the Starjammers lead by Gladiator. Vulcan is an Omega level Mutant, and can absorb immense amounts of energy and then turn that energy into a weapon. Vulcan has faced the entire X-Men and even the Starjammers (a galactic police force created by the Shi'ar empire), it took Havok to absorb all of Vulcan's energy to finally defeat him.
Norman Osborn
One of spider-man's first villains to create such an impact on his life, Norman Osborn is an evil genius. Though his son Harry Osborn and Peter Parker are best friends it didn't stop Spider-Man from having trouble with Norman's evil personality, Green Goblin. Goblin threw Peter's girlfriend Gwen Stacy off of the George Washington Bridge, forever making Peter doubt whether Goblin killed her or he did when he caught her on the way down with his webs (This event is highly disputed as one of the most pivotal moments in Spider-Man history). Not only that but recently in comics he took up the moniker Iron Patriot when he was the head of HAMMER Industries and the leader of the New (Dark) Avengers after the Skrull Invasion made the people of New York lose trust in the Avengers.
Baron Zemo
Evil runs in this families blood. Baron Zemo I was Captain America's nemesis during WWII, and was actually the one who sent off the plane that killed Bucky and sent Cap to his icy coffin for over 60 years not Red Skull. Baron Zemo II is the first Zemo's son, and the mask they both wore is seared to his face this time after a battle with the newly awoken Captain America. Leader of the Masters of Evil and connections to Hydra with the Red Skull, Baron Zemo has been a mass murderer for decades going through generations.
Doctor Octopus
One of Spider-Man's most famous villains and perhaps the most persistent, Doc Oc has given him trouble for decades. Otto Octavius was a Doctor of Science, so Peter Parker thought of Otto as a role model until he had four arms seared into his spine during an experiment. His brain had to compensate for the extra limbs which is the running theory on why such a well respected scientist turned evil. Founder and leader of the original Sinister Six, Doc Oc has done countless things to stop spider-man, which has drawn attention towards the Avengers as well. Recently he was inside of Spider-Man's body.
The Beyonder
Member of the Otherworldy race called the Beyonders, he is literally the unverise in the shape of a human being. Thinking that all humans/superhumans lack certain emotions lost to him, he believes that all of them are 'incomplete'. He can easily changes states of matter and due to his Nigh-omnipotence, he has limitless strength. With no real side, he has fought and helped both heroes and villains. The ability to create and destroy anything he chooses makes him one of if not the most powerful being in the universe (or in himself).
Annihilus
The Lord of the Negative Zone and the Living Death Annihilus is one weird villain. Scared of only one thing: Death, Annihilus owns the Cosmic Control Rod which drastically expands his life. Living in the Negative Zone, another universe found by Reed Richards, he lives there commanding his bug like army to do his bidding and to find a way to take over earth. He was once killed by Nova, after he shoved his arm down his throat and then pulling out his organs. He was brought back by the Kree. During a fight with the Fantastic 4, the Human Torch destroyed his entire army but was left behind in the Zone. Annihilus wanted Torch to open the portal back to Earth but refused to, resulting in his death.
Magus
The evil counter part to Adam Warlock, is as crazy as he is scary looking. Though Adam and Magus are one in the same being, they are completely different from each other. Magus is the crazy and Lord of Chaos part of Adam Warlock. Warlock has only been able to really keep him at bay when he was in possession of the Soul Gem. Magus possesses vast cosmic powers and was able to defeat Thanos single handed.
Wilson Fisk
CEO of Fisk Industries, Wilson Fisk is the crime lord that rules over New York. With millions of dollars and plenty of henchmen to do his dirty work he is not an easy man to stop. If only he could get Spider-Man to stop saying fat jokes, when in reality he is 400 lbs. of pure muscle and can wrestle extremely well. Daredevil also seems to be there to give him trouble (see Daredevil on Netflix).