I can never pick just one for these things. I'll really try to limit though.
Lilo and Stitch. It's just a perfect story, great style, and just an all around good time. Who didn't love something cute and fluffy?
Adventure Time. Great adventure, basically a show that animated what my imagination felt like as a kid. I'll get to play a DnD game set in Ooo one day (actually want to use Wyrmwood Wand but that'll be even less likely).
Angels Egg or Neon Genesis Evangelion. I basically marathoned them back to back in college. Everyone needs to watch something that leaves them going "wtf" at least once in their life.