A good utau is determined by the user.
Personally, I prefer stronger, slightly robotic utau voicebanks. Typically monopitch in CV or VCV. I'm learning to use CVVC and I'm starting to like it more, I'm still not 100% familiar with it. I also really like powerful, solid or clear voicebanks. Another thing I really like is a good low range. Most songs are pretty high, so a good low range doesn't really matter, tbh...
I tend to stray away from multipitch banks because they're huge, and don't really do much but take up space. I also don't really like whisper banks, or kire banks. I enjoy being able to hear the robotic aspects of utau, it's almost comforting. I also tend to stay away from utau in languages aside from Japanese. I'm useless in ust/song making, so every ust out there has already been used by every English utau out there and it's pretty boring. I also don't have a grasp on how English voicebanks work, and none of my own.
I am also one of those users who prefers to use their own utau.
Long story short, create whatever bank you want, someone out there is bound to like it.