They've cast quite a few villains - Alvida, Buggy, Arlong. I don't think Captain Kuro has been cast, so he might not be in it. Likewise Don Krieg. So it might just the three of Alvida, Buggy and Arlong. They haven't even indicated they are going to feature Captain Morgan despite casting Helmeppo.
I'll be hard to pick a favourite until the show drops. Depends on how they are realized too. Arlong is the most vicious and brutal out of the three.