I'd have to say either Forest Gump or Saving Private Ryan I pretty much like any of his movies though.
Oh man Tom Hanks has so many great movies. Just to list a few of my faves Toy Story(1, 2 and 3) Castaway The Green Mile The Terminal He's good in just about everything.
Rain Man and Forrest Gump he's really a good actor when he acts like someone who's challenged. I think that he also helped "normal" people to understand those people and their potential