Not gonna do the last one as it's apples and oranges! As for worst, I haven't seen them all but I saw a bit of the Earthsea one and it wasn't great. Oh, and if it counts then some (though not all) of the dubs make me want to switch off right away. :x
Have you seen them all?
Nope... maybe about half overall?
Favourite?
I have a few that I like for different reasons... I have a huge soft spot for Kiki's Delivery Service, and I've found that several of my illustrator/creative friends do as well. I don't think it's coincidence given the themes of the film. Love Mononoke but it's both so long and so intense at points that I can't watch it often. Loved Whisper of the Heart and I liked The Wind Rises way more than I thought I would. The most infinitely rewatchable one for me is Howl's Moving Castle - it's different from the book but it has such a special something of its own ( *whispers* I might actually like it more than the book...), and I can watch the dub or the sub comfortably enough. Total loveliness and comfort viewing.
Edit: Forgot to mention Spirited Away - I think I saw it during all the hype and it didn't really settle as one of my favourites; maybe partly due to all the hype, and also to me it felt a bit like "then this happened, then this happened..." but I watched it properly again a few months ago for the second time and I felt what it would be like to watch it as a child - it would have been one of those films like Labyrinth or The Neverending Story, which just pulled me in with their weirdness, scale and vulnerability and completely bewitched me. I'll only know once I watch it a third time, but it might be edging towards the favourites list now.