Oh that's tricky. My favourite Stand would have to be Aerosmith from Part V. It's unique because it's literally a plane. Or maybe Crean Starter from Part VII? I think I like the idea of stands as stuff and no a Guardian. Since they tend to be rarer it's a novelty when they show up!

EDIT: I forgot Scary Monsters! I won't spoil it, but it did make me laugh in a "haha! That's so AWESOME!" way when I first saw it.