Hoffman -- he's played everything. Lenny Bruce, Willie Loman, Rain Man, mild-mannered types (The Graduate, Kramer vs. Kramer, Outbreak) gangsters (Billy Bathgate, American BUffalo) corporate sleazeballs, kids' movie characters, the cross-dressing guy in Tootsie, you name it.
Hanks is up there too of course, but he just hit his Renaissance -- Hoffman's been at it non-stop for over thrity years.
I'd add de Niro for his willingness to do pretty much anything for a role, although he plays the same basic guy in just about every movie -- but the things he did for Raging Bull and Taxi Driver were pretty out there even if they weren't definitively versatile.
Phaedrus