1. Howl's Moving Castle. Just glorious looking, and the music's fabulously fantastic. I've several versions of the soundtrack on my computer/iPod, and I know every single song by heart. I've listened to them so many times, if they were on CD's, they'd have broken.
2. Spirited Away. Again, the animation and art direction's insanely top-notch. It's crazy. Also, the story really moved me. The characters are funkily out-there and loveable.
3. Hmm. Totoro. I just can't help it, it's a classic. The animation and art, though somewhat crude when compared to Ghibli's later films, are still amazing. It's got a totally unique light-hearted feel to it, and some catchy music, too. It's just utterly charming.
Mononoke's a close 4th. Loved that one, too.