If you're talking about 3-D animated films, then I'm going w/ Finding Nemo. Shrek and Toy Story are right up there with it, but I found Finding Nemo's story incredible.
Now, 2-d Animation... wow.. there are just way too many to choose from... but I'm going with Katsuhiro Otomo's AKIRA. Made in 1988, it's probably one of the most infulential 2-d animations ever made. It is rated R, so yeah... if you're too young or can't handle it... or something.. don't watch it. But really, Otomo brought Anime into the limelight with AKIRA, so I suggest you watch it if you haven't already... OH yeah, and Kaneda (main character) has one of the coolest FF cycles I've ever seen.
(oh, and the cycle pictured is just a full scale model. There is no true working model of Kaneda's bike in existance, but many FF cycles have been inspired from this design.)