I'd very much like to become a dolphin, or a whale. It isn't about size so much as it is about being one with the ocean... I'm very much in love with the ocean, and that's largely why I'd like to become an Animagus. I want to be able to just walk into the ocean, change, and swim away for an afternoon, see things that no human will ever see.
Oh gosh, is that really selfish and horrible to say? Because I don't mean to be... I think as someone who can appreciate the ocean in that way I'd be better equipped to work for conservation and such.