I have to go with Schindler's List, but it's close.
Forrest Gump is without a doubt a tremendous film, but it doesn't have the impact on you Schindler's List has.
Schindler's List is one of the greatest movies of all time and my favorite Spielberg film outside of Empire of the Sun. There are other movies I would vote over this, but Forrest Gump, while a great, great film, just isn't one of them. IC has already wonderfully gone over what makes Schindler's List so spectacular, so I'll just add that Schindler's List was also Spielberg finally walking away from his Peter Pan, shark, dinosaur, alien and other kiddy fascinations, and he did a hell of a job doing it. Most writer/directors who make that transition, more often then not, fail miserably, but Spielberg showed his range with this film and it couldn't have been better for him. Its ten times the movie The Color Purple is in my opinion and like I said, one of the greatest movies of all time.
Also, and this is just my extreme dorkiness coming out (a lot just might consider it gay, lol), but one of my favorite scenes in television history was brought to me mainly because of Schindler's List. I am a huge, huge Dawson's Creek fan. Now, for those of you not familiar with the show, the main character Dawson is an inspiring film maker who heavily admires Steven Spielberg. Now, in the show's pilot, at the very end of the program, there is this tremendous moment that revolves around the characters of Dawson and his best friend Joey (who is a girl, by the way; she just has a boy's name) where they're talking about to growing up and Joey says, "We're growing up, that's all. I mean even Spielberg outgrew his Peter Pan syndrome." She says this while very quickly pointing at a poster (which you don't see; also keep in mind that it seems Dawson has nearly every Spielberg film poster ever made hanging all around his room). So anyway, they get in this argument about where Joey says they're getting too old to talk about "everything" anymore and too old to sleep in each other's bed every night, and Dawson completely disagrees with her. So, to prove her point, Joey says if you can tell me anything, how many times a day do you *********e (she actually said "Walk the dog" but *********ing is what she meant by it). Dawson can't answer so she hops out of the window and starts to go home. Dawson then gets up and bangs his head on the Schindler's List poster, which you now clearly see is the poster Joey pointed at and is completely symbolic of people growing up, and then heads for the window to yell out the answer to Joey's question. It is an awesome fucking scene. Well acted, and revolved completely around about how this movie, Schindler's List, was Spielbergs ticket to growing up.
Schindler's List