An UTAU's voice must have a certain amount of two things in order to be an aesthetically sound voicebank
1. Depth- An UTAU's voice must be recording in a way as to invoke a feeling of the vocal you are trying to capture. Depth is a quality that makes a voice more or less vivid. Triphone/VCV voicebanks usually have much more depth to them the Diphone/CV voicebanks.
2. Characteristic Vocals- Your UTAU most by the embodiment of your voice, so don't try to mimic another UTAU's voice unless the UTAU is a product of voice acting. Record a voicebank with confidence in order to make an UTAU unique.