Ah. I've already a very, very hard time choosing favourite Vocaloids. But utau's ah. There are so many good utau's, talented utau users, or beautifull designs. I always state Matsudapoinne as my favourite, but that's just because for some reason I think it's somewhat awkward when a user can read that their utau is my favourite. I know that's ridiculous, but I'm always afraid to look like a stalker.
I'd say my favourite Vipper is Ruko female (though her VCV sounds kinda manry, but whatever). And then, I just don't know anymore, I don't really have a favourite utauloid, I've just periods of being 'into' a certain voice, like, a few months ago I couldn't stop listening to Toby Osbourne and Mimi Yorune. But, ah, as I said, it changes a lot, I think I'm atm into Sora Denatsu, Yume Shinya, Konpekino Sora and Kyone Killy.