Can't do just one.
But Miyazaki - Mononoke/Spirited Away/Totoro
Sword of the Stranger has the best sword fight in anything ever.
Patlabor 1 & 2 (ignore the third) - Both are pretty terrible at showing Patlabor's biggest strengths, but they're both great films, and as time goes on I like them a lot more than Ghost in the Shell.
Girl Who Leapt Through Time is wonderful.
Macross: Do you remember Love... Good luck watching that one legally. Almost recommend the Macross Plus movie, but the OVA is what you should watch.
A newer one I adored would be Promare. It's everything I loved about Gurren Lagann, and is fucking insane. It's pretty much Burning Rangers: The Anime.
@sweep: I rewatched this recently and was a little disappointed by it. The animation and the action sequences are amazing, and it's hard to complain about getting to spend time with the cast again, but at two hours, which is longer than any single story in the show, it really, really drags. Vincent is sub-Vicious, and most of the cast are really, really underused. The animation is so good though, it feels a little more like a showcase for animators at times. There's so many scenes with little details that look cool but just slow everything down.