scott
Usage is most important and all the vippers can be used well, I guess I'd say my least favorite are Rook and Oniko (though neither are bad) as all the others are pretty even, with Ruko and Ritsu having an edge with the sheer variety of voicebanks available.
With all UTAUs ever, haha, too many to say. And having lots of favorites in my mind is a good thing - there's a huge variety of voice types out there !