You can choose any that you feel like shows the best strength of the UTAU VB itself, for example...
I use Romeo and Cinderella for my CV (original) back then because I want to show the opposite of power. It gives a softer tone.
Apriloop for my MALE voicebank to demonstrate the power/deep tone of a sad song.
RYN1T3 was to show a cutesy/rapsy voice through Magician of Love.
My VCV demo uses SCALE BLASTER to show a softer side as well as a deeper tone that makes it different from the original cover, while EL3CT0's demo uses It's Not My Fault to present the deep/power vocals that the original song didn't use.
Picking a demo song for your UTAU is about deciding what you think can represent the best for the bank itself. You aren't restricted to only doing one demo song as you can do multiple however doing only one is technically up to your choice as you can return back to redoing the cover in the future if you're not satisfied.