Daniel Day-Lewis, this shouldn't even be questionable. Ed Norton, Leonardo DiCaprio, Jeremy Irons, and Matt Damon also stand out on that list, but even they have a few performances a piece that aren't anything to write home about. Daniel Day-Lewis has never put in a bad performance in his entire life, and his list of roles is simply unparallelled.

De Niro and Pacino shouldn't even be on that list. They've both been phoning it in so long that their past glories are very heavily tarnished. They both should have retired 15 years ago. Tom Hanks is also someone who would have been a stronger challenger several years ago. He hasn't been putting in negative performances, but he's definitely resting on his laurels. Hanks has pretty good range, but doesn't seem inclined to push himself.

Omissions that should be on that list (though still wouldn't get my vote for 'best'):
Peter O'Toole
Christian Bale
Brad Pitt
Johnny Depp
Benicio Del Toro
(as noted above) Michael Caine
Ben Kingsley
Geoffrey Rush
Anthony Hopkins