I'm going to assume that you have subs that aren't neck deep into the fandom and the combination of all that it entails (music, vocaloid, anime). I'm also going to assume that not all of them are interested in Japanese (speaking it, watching shows completely in it, etc.)... Therefore, my picks will be...
CZloid - the 'ur example' for English UTAU. Assuming that the majority of your audience will be English speakers, best to start with an utau in a language they'll understand.
Crina - has a crap ton of banks for different expressions/voicetypes. Good for showcasing versatility for those who assume synth = monotone and robotic
Namine Ritsu - provides the "famous/everyone and their grandmother loves ___" factor as well as being more on the realistic side than the aforementioned Teto (she's famous, yes, but I find her voice to be in the love it or hate it category because even though it's done by a human, it's voice acted/pitched and doesn't sound natural which can be a turn off if you're trying to showcase the best of what UTAU can offer).
Any given voice with multiple languages to it is definitely a plus. Forgive me if I name my own among these:
Hanami (Japanese, Korean, Mandarin)
Tsukine Sayuri (Japanese, Korean, Mandarin, English)
Kanochi Neko (Japanese, Korean, English)