Akira. Pretty much the only anime where I don't want to read the manga, incase it wrecks the movie. I know, it's normally the other way around, but meh.
Kimi Ga Nozomu Eien had a great ending. It's hard not to spoil, but it was just so well done. Really sad yet happy at the same time.
Now and Then, Here and There was also great, I was really blown away when I first saw it. Even if the episodes themselves weren't always perfect, and the main character is more annoying than anything, the ending really made the series.
Oh by the way, it's my first post on the forums. Hai! ^^