Loki was enjoyable, but his scheme's didn't seem very well planned out. Zemo was sort of the opposite, his plan was laid out (if somewhat over-complicated), but he just wasn't really fun. I didn't like Nolan's Joker, though I recognize I'm in the minority there. Same with Doc Ock, thought he was okay but his plan was dumb.
I found Ultron amusing, and his plan was foreshadowed and explained, but then it was still a bit bland overall.
I guess Doomsday was okay as a monster, but dear god Luthor was the worst thing I've seen in a movie in a long time. I'd rather have Green Lantern's villains recurring.