Honestly I like a lot of voice types, but my favorites are really soft female vocals. Which is why I'm severely over-attached to Momone Momo. I feel like her voice exemplifies my favorite type.
I really like things to sound clear, though, and only a few robotic voices are good in my opinion (I have a soft spot for Defoko and Teto's old, robotic VB from songs like Song of the Eared Robot). Powerful, mature female vocals are also really cool, because they can cover stuff Momo wouldn't dream of even trying to sing. The only thing I dislike is overly low- or high-pitched singing, unless it's denpa, in which you get a free pass. It depends on the song a lot.