Before 1970: Fantasia (1940). This is actually the only animation film I own on DVD, though I like most of the early Disney films. There's quite a few of the one in the '60s and the more modern ones that I've never even seen (101 Dalmatians, Jungle Book. etc.) Fantasia, though, is perfect. The animation is gorgeous, lush, exquisite. The music is wonderful. I grew up on classical music, and I'm sure this movie shaped the way I hear music, and even the types of classical music I like. I prefer my music to evoke images. This is also part of the reason I love orchestral soundtracks so much. My favorite segment of Fantasia is Beethoven's 6th Symphony. I love the flying horses, unicorns, centaurs, and mythological setting. The only section that I don't love is the sort of intermission bit on notes/sounds. Though, man, every time I see the Night on Bald Mountain segment as an adult, I'm surprised that it didn't scare my pants off as a kid. It's frightening!
Post 1970: Cars (2006). This is my hands-down favorite of the Pixar films. I do not tire of watching this one. You know it's bad when your nephew wants to watch something, and you're telling him "Cars! Let's watch Cars!" just because you want to see it yourself. I find this ironic too, because I don't like car racing generally, and I also cannot stand Owen Wilson in anything I've seen him in -- but I don't mind him at all as the voice of Lightning McQueen. He suits the role. I love the story of this one, the little life lessons. I love love love Doc Hudson, love the whole tractor-tipping, love how the beautiful scenery awes McQueen. Love the town of Radiator Springs and how they spiff it up. Love Mater and Luigi and Guido and, really, all the characters in Radiator Springs. It's just a very satisfying movie, and it doesn't get old. (The Cars dvd also has my favorite Pixar short film: One Man Band. I haven't tired of that one yet either!)