(In no particular order)
George Clooney as Batman
I actually respect Clooney as an actor (for the most part) but Batman just wasn't the right role for him. Plus, the Bat nipples and incredibly weak script degenerated his performance quite a bit.
Halle Berry as Catwoman
Berry was at the bottom of my list of people who I want to see play Catwoman. Anyone who thought she'd be good was probably dropped on their head as a baby. Also, I don't know if their decision to make her character separate from the Batman version was a good thing or not.
Arnold Schwarzenegger as Mr. Freeze
He looked decent enough in the suit, but his consistent puns and accent (not that I'm prejudiced concerning accents) made his performance very, very forgettable.
Tommy Lee Jones as Two-Face
He seemed more like a Joker spin-off with merely pain on half his face than anything else. Two-Face is supposed to be a very dark and destructive character, not a laughing douchebag from a Saturday morning cartoon. The very performance of the character was one of the things that destroyed my liking of Batman Forever.
Topher Grace as Venom
He wasn't too shabby as Eddie Brock (albeit he didn't seem near as tough as his comic book counterpart), but he didn't seem to fit in as Venom (despite their mostly getting the Venom design right).
Terrence Howard as James Rhodes
Did anyone else notice that Howard didn't seem to look natural in Iron Man's setting? Might be just me, but I'm seriously glad they replaced him with Don Cheadle for IM2.
Chris O' Donnell as Robin
I thought Robin was a young teenager? O' Donnell was too old for the part, enough said. Also, his personality in both Schumaker films was mediocre at best. No wonder a lot of people don't want to see Robin done right.
Anna Paquin as Rogue
to me, Paquin seemed too bland as Rogue. She's cute and all, but I just can't quite place my finger on it...she was probably too shy-acting in the first X-Men.
Jeep Swenson as Bane
I'm kind of surprised I even mention Bane in this, his role in Batman & Robin was that forgettable. He was downplayed to the level of a gorilla...yeah, it was that bad.
Gene Hackman as Lex Luthor
He just didn't seem right for the role. I mean, he had the looks to a degree, but produced a pretty 'meh' feeling by the end of the first Superman film.
Well, there's my list. Next edition will be on performances that are in between the best and the worst. As always, sound out any thoughts/suggestions in the comments below.