Ultron was the worst AI bad guy I have ever witnessed.
I think there have been some worse ones in more low-budget productions, but if you're talking about mainstream AI then yeah, he's not that great compared to some of the best, but I actually thought he was fairly effective and sympathetic villain brought-down by some quips.
The problem with ultron was he suffered from Whedon over-doing the quips and the movie missed chances to show him being as threathening as he should have been. His 1vs1 fights with IM, Cap and Thor should have been a lot more one-sided (IM fight should have been last to make the final battle more personal) and he should have killed QS more directly. Other than that he was okay. Great voice-acting from Spader, nice monologues, fairly interesting creator-creation relationship with Tony.
But yeah, Pierce, Winter Soldier, Zemo, Loki and Red Skull were good. I also liked the Mandarin twist and reveal that Killian was using him to hide his mistakes and play both sides in the war on terror. Still think IM 3 has one of the best scripts in the MCU.
I think one of the reasons I did like Ultron was because of the Stark / Ultron relationship. It just seemed so well-developed to me, and was very interesting.
Red Skull suffered from the hero emphasis over villain. He should have been a great baddie, especially since they cast Hugo Weaving in the role, but instead he sorta become generic. Not badly done, but just kinda there. I did think his true face scene reveal was very well-done though.
Iron Man 3's twist is something I've grown to appreciate, and I think that film is a bit underappreciated in general, however I really can't say I particularly cared for Killian as a villain. As an actual character, he was just a bit too "lame" for me frankly.
Marvel's not been particularly great with their villains. I will say that I think that some of them do get the job done as evil ha-ha badguy (well okay some of them aren't blatant), I just wish they explored their baddies more. Maybe make more villains with more sympathetic backstories like Zemo or even Ultron, after all there is a lot to pull from the comics, but I'm assuming the writers are often restricted by the studio, so oh-well.