Well, I've only been behind the management of one UTAU, and that's eseph, who just released the other day.
Other than that, I've mostly worked on projects for other people's already existing UTAU, such as Shione Lt.'s English bank.
As for my favorite, I'm not sure I have one particular favorite, but I definitely have some I like the most -
adrian - Specifically their "medium" voicebank. There aren't enough good things to say about this voicebank. It's a triphonic, dipitch, multilingual voicebank, that specializes in English, and they sound divine. Quite possibly one of the best English voicebanks ever made, not even just in UTAU.
Peiton - Specifically his "Good Vibes" bank. It just... has so many things in it. There's so much to play with, and if I don't have a vocal in mind for a song, he's my initial go-to.
Mawarine Shuu - I don't use Japanese voicebanks that much, but when I do, that's usually the first one I play with when making .ust's. Seriously, his range is utterly absurd.
Mine Laru - He just has a really nice voice. I really hope his ARPAsing bank releases soon. I already know that he has two English banks already, but they're pretty hard to use and require a lot of adjustment to make sound good since their oto.ini's are a bit weird.
Gekiyaku - Call me basic, but she has a very unique voice, and a decent English bank, sue me.
Anna Nyui - Originally, I though her voice was kind of boring, but I've really started to warm to her as of late. I've always admired her English-NEO- bank, and I'm also a big fan of the "raw" voicebank, since it does some rather interesting things. Then her recent "DIVA" bank is also very interesting. She's also getting a Spanish bank, which will certainly be interesting - we don't get a lot of Spanish banks with Japanese voicers.