Hanks - He literally carries the whole movie on his shoulders. Philadephia, Forrest Gump, Cast Away, Saving Private Ryan. Even his Toy Story stuff is by far the best anime voice performance ever. I don't think any leading man in Hollywood has come even close to his class. Sadly though, lately, his choice of movies have been pretty pathetic.
DeNiro - For me the best performance ever in a movie was from him in "Taxi Driver". All his Scorsese movies.. Mean Streets, Raging Bull, The King of Comedy, Goodfellas, Cape Fear, Casino. Every single one of them is a classic. The best actor-director team ever.
Al Pacino - Brilliant, can express extreme emotions probably the best out of all of them but I don't think he can play every role. His "Godfather II" role was the bomb for me. Throughout the movie he appears cold as ice and than in a couple of scenes he just explodes, esp. the one with Diane Keaton when she told him about her abortion. Also, that "Dog Day Afternoon" scene when he is talking on the phone with his gay lover. It can't get any better than that.
Others who I like, Brando- who started this whole "method-acting" movement. Before him the norm was over the top theatrical acting personified by Laurence Olivier. So he is like the God of acting.
Philip Seymor Hoffman - absolutely love this guy whether he is in a lead or side role. Brilliant stuff from him in "Before the devil knows you're dead", "Doubt" and all his PT Anderson movies. I've read he is great in the soon to be released "The Master" loosely based on Scientology's creator Ron Hubbard.
Leo DiCaprio - If anyone deserves an Oscar, he is the most deserving. Even as a child his work in "What's eating Gibert Grape" and "This Boy's life" can be compared to any acting role by an adult. Loved his role in "Revolutionary Road" which was criminally unappreciated. Great acting in "Inception", "Body of Lies" and "Shutter Island". Best of the young blood in Hollywood.