Jack Nicholson by far. He's a Renaissance actor with such a versitile range of roles from The Shining, Wolf, As Good As It Gets, Anger Management to About Smidt.
I voted for DeNiro.He is the greatest character actor I have personally witnessed,being able to play a convincing invalid,psychopath,toughguy,comic,etc...He has the widest,most convincing range of any actor today IMO.