I put "best" in quotations because I'm going to give a specific type of qualification for consideration. By best I mean, in your opinion, who is the actor who has done the most movies in which he played a character that you thought "man, so and so was fantastic in that movie." So, someone who has done one fantastic, blow you away movie but never really done anything else wouldn't qualify.
It's funny you said him (and that he was the first response) because seeing Lethal Weapon last night is what made me want to pose this question.
He's not particularly my favorite, but for me DeNiro would take it. He is so powerful and just so versatile.
Actors I enjoyed watching: Tom Hanks Daniel Day-Lewis Kurt Russell Russell Crowe Johnny Depp Ian Mckellen Mel Gibson Jodi Foster Jennifer Connelly
I don't know if he is the "best". but for me, I look forward to anything Phillip Seymour Hoffman is in. as for females.....again, maybe not "the best", but I seem to enjoy quite a bit of Jodie Foster movies.
Heath Ledger... even though he only had one great performance.. that performance is the greatest of alltime.
Since his death, and going back to watch some of his films I never saw, I have a new appreciation for Paul Newman. He's pretty fantastic in his movies.