In many ways an excuse to post this which I wrote about Robert De Niro: http://permanentplastichelmet.com/2011/12/12/has-robert-de-niro-now-been-crap-for-longer-than-he-was-great-and-does-it-matter/
But I am interested, seriously.
For me it is De Niro, because I can't think of anyone else who at their peak could disappear inside a role so completely. Raging Bull's probably his best, though The King Of Comedy is probably my favourite. Amazing, underrated film.