There seem to be enough Ultra Beasts running around Alola to have a favorite. Since there are eleven (twelve if you count Necrozma), it's been a harder decision for me than it should ever have been. I personally love Blacephalon, and I love using it. What's your favorite?
I like all the Ultra Beasts but I like Naganadel the most. It’s a Dragon-type and it’s a very powerful Ultra Beast as it can set up with Nasty Plot and then land a powerful Devastating Drake from a Draco Meteor base. It’s also fast and has high Special Attack to accomplish that role.
Nihilego, I like its unique typing (even if it makes it very weak to Ground moves) and high Speed and Special Attack. I like Pheromosa, Naganadel, Kartana, Celesteela and Stakataka to a degree, too. Guzzlord, Buzzwole and Blacephalon creep me out, though.
The Ultra Beasts aren't my favorites in general - they feel overdesigned to me. However, I have a weird liking for Buzzwole that I can't explain; I named mine "Mad Gainzz" and I laugh when I use him in battle.