Michael Keaton. He's the only one to do the 'Dark Knight' and Adam West both justice. I remember when it was announced that he would don the cape and cowl for the Tim Burton Batman film. Everyone hated the idea, they couldn't see such a comic actor, being Batman. And I'll agree, it seemed ludicrous at the time. But when the movie opened, I was blown away at just how good a serious actor Michael Keaton could be. It's not an easy roll to fill, that's for sure.
Big names like Val Kilmer and George Clooney definatly showed that it's very easy to bomb big time in the roll. Plus the other casting in Batman/Batman Returns was great. Nicholson as the Joker, Pheiffer as Catwoman, Deveto as Penguin. And then it goes downhill with Jim 'Overacting' Carrey as Riddler, Arnie as Mr. Freeze, and Uma as Poison Ivy. Tommy Lee Jones was ok as Two-Face, but his roll was cut down to almost nothing thanks to Carrey's Riddle.
By the way, MTN...this might help some:
Adam West - Campy 60's Batman TV Series/Movie
Michael Keaton - Batman and Batman Returns
Val Kilmer - Batman Forever
George Clooney - Batman & Robin