Tough one, but my favorite is Catch me if you Can, followed closely by Saving Private Ryan.
I just loved Catch me if you Can. Leonardo DiCaprio, after only Mark Whalberg, is in my opinion the best actor in the world. Him and Whalberg are just on another level, and Tom Hanks is just amazing as well. Catch me if you can was more than just a really fun, awesome movie, it was a really GOOD movie. It's one of my all time favorites (plus, the scene with Jennifer Garner is just so awesome I don't even know where to start).
I actually went to see the real Frank Abagnale Jr. (the guy that DiCaprio played) speak a few years back when he came here to Vancouver. Man, he had some stories. AWESOME speech.
I loved Saving Private Ryan as well. To me, that is the best war movie ever made. It was just so...real, and again, Tom Hanks was beyond amazing. Also, I'm a sucker for anything Matt Damon is in, so that helped.
(I know it isn't a movie, but Speilberg and Hanks created an HBO miniseries called Band of Brothers which is just amazing. One of the best T.V. series I've ever seen, hands down.)