Aldrich Killian: Finally, Iron Man faces someone who isn't just another guy in a suit. Killian really had a brilliant plan, no matter how you feel about the Mandarin twist, and he fit perfectly into the story Shane Black was telling with Iron Man 3. Plus he breathed fire. 8/10
Malekith: You have to give him some points for looking cool, but I argue that Malekith takes the cake for the biggest waste of a villain in all of the MCU. He really doesn't do anything memorable, heck, his henchman Kurse has next to no lines and is more memorable than him. Despite his great actor and design, Malekith managed to be the weakest link in an already weak movie 4/10
Alexander Pierce: When Robert Redford signed on to the part, you knew it would be good, and he certainly lived up to the hype, proving a much better Hydra head than Red Skull managed to be. And with him, his death was fitting and very well executed, unlike so many needless dead villains in the MCU 9/10
Ronan: Much like Malekith, Ronan had a great actor and a great costume to help him out, but never managed to be all too impressive, and was done a huge disservice by the film's final act. But he at least managed to sneak in a few more memorable moments than the Dark Elf. 5/10
Ultron: Well, Ultron is tough to judge. On one hand, he's not a bad villain. Spader and Whedon gave him a unique and memorable personality. On the other hand, by the time he reached the climax, the film was veering painfully close to the original Avengers. At the end of the day, AOU just never fulfilled on the threat that Ultron SHOULD be. The HISHE video for the movie showed a more threatening villain, for Pete's sake! 7/10
Yellowjacket: You can't get a much more looser adaptation than this. Darren Cross of the film has little to do with either his comic counterpart nor that of his alter-ego. But he boasted one of, if not THE, best villain costume in the MCU, and Corey Stoll delivered a brilliant performance with what he was given, which was tragically little character development and a presumed death scene which was massively disappointing. 7/10