I've only seen Nausicaa, Totoro, Mononoke, Spirited Away, The Cat Returns and Howl's Moving Castle which is the newest, I just saw it last weekend :P

All of these have been really great, especially the Cat Returns, 'cause it has cats in it ^^; No seriously, Howl's Moving Castle is by far the best along with Princess Mononoke. It's kinda hard to even try to explain why, they just are :D Maybe it's because they provide so much to think about that they never really leave you bored, not once during the movie. *shrug* anyhoo, everyone should go see Howl's Moving Castle (I think it's "Hauru no Ugoku Shiro" or somethin like that in japanese :3).

By the way, its "Ghibli", not Gibli. :)