Just to be different, I'm gonna have to say I'd want to be an Arn.
Sure, they always get ridiculed because they're cowards or whatever, but they are immune to Yeerks, and can create basically anything they want out of DNA. Yeah, I know it's 'wrong,' but who wouldn't want to
create their own pet? I'd make a Compsognathus and cross it with an Andalite and it would be so cute and fuzzy and adorable and badass. And I would name him Andy.
Too bad the Arn are extinct, huh?
. . . OR ARE THEY?
. . . Lol, j/k, they are.