I tend not to have a consistent favorite. However, movies from Stanley Kubrick's filmography do tend to routinely find their way near the top. Particularly 2001: A Space Odyssey and The Shining.
I also really love Akira, from Katsuhiro Otomo. That one in particular really broadened my interest in the medium when I first saw it, and I can re-watch it any time and become totally engrossed in it.