the first UTAU that comes to mind is yamine renri, her voice is very soft, calm, and she's the literal queen of vocal fry. but if you're looking for something that isn't the A list celeb UTAU :))
personally i've been having a lot of fun with saccu's (gahata meiji's vp!) new UTAU wistaria. technically she's still in beta, but i haven't found any game breaking issues. she's a 4 pitch vcv, soft n cute. she needs more users since everyone's picking bibi over her!!! >:/
i'm not too much of a fan of cutesy voices, but these are ones i've used and really enjoy!!!