You guys are across the board with what qualifies great acting. There's a difference between an actor that can nail a role and an actor that can nail any role.

When it comes to an actor who can become the true incarnation of ANY character, Daniel Day Lewis is second to no one, and actually far ahead of the game. Philip Seymour Hoffman is probably number two. I'd like to put Bale third, but that's debatable. I put Norton in top ten famous Hollywood actors, and probably DiCaprio as well.

Guys like Eastwood and Sam Jackson and Will Smith are good, but not that multi-dimensional, and thus cannot be considered in top lists. Denzel is borderline. He has done a few great roles, but if you've seen three of his movies you've seen them all. He is the same dude more often than his fans will admit.

Daniel Day Lewis is NEVER the same dude, and his roles are perfectly performed while being often antipodal in character. Just look at the differences between My Left Foot and Gangs of New York and In The Name Of The Father. The characters are as different as 'green' and 'A#', yet he embodies them all. Can't wait to see There Will Be Blood, will be seeing another entirely different DDL.

Lesser known actors like Chris Cooper and Ethan Hawke are better than most actors mentioned in this thread. However, it's hard to say exactly how good some actors are because they often gravitate towards roles that they're exceptional at. Actors like DeNiro, Crowe, and Pacino are awesome, but they tend to not pick roles that require a ton of adaptations. But they seem to be able to, and we saw as much by DeNiro in Awakenings.