Either Lycanroc or Kommo-o. Both of them also have really nice shinies, even if a lot of people don't like Kommo-o's for some reason.
Vikavolt. I’m not even particularly sure why, it’s just one of my favorites. It has an awesome design and concept. I used one when I played Ultra Sun and it was one of my strongest Pokémon.
I like it, but it isn't my absolute favorite. It's still pretty cool though, I really like the silver. I think I like it more for its stats, though- the special attack is a killer, and it has some pretty decent defenses, too.
Incineroar, Salazzle, Kartana, Golisopod, and Midnight Lycanroc for sure! I really love their designs, shinies, and had fun using them in SM/USUM. They are also some of my favourite Pokémon ever, with all of them being in my overall top 15. Honerable Mentions: Turtonator, Lurantis, Bewear, Mareanie, Popplio, Brionne, and Decidueye.
I love alolan raichu. My heart melted when I first saw it, and I used a shiny one in my UM playthough. If we're talking about the brand new Pokémon (not counting alolan forms), then my favorite is decidueye. Mine were great physical attackers.
You could probably already tell because of my avatar, but Alolan Sandshrew/Sandslash is my personal favorite. I also like Alolan Raichu, Mimikyu, and Rockruff.
Primarina is probably my favorite Alolan Pokémon. The design is gorgeous, and it's good in game as well. I'm also quite partial to a few of my other team members like Tsareena, Bewear, and Salazzle.
Celesteela, Ribombee, Alola-tales, Tapu Fini, Araquanid... there were a lot of great mons introduced in these games that I really like. Too hard to pick a favorite, but those were some that came to mind.
Poipole is precious and must be protected. Blacephalon is a weird exploding Dippin Dots clown and I love it.
Alolan Vulpix and Ninetales were wonderful additions and were what got me excited for the games! I also really like all the forms of Lycanroc, because I'm a sucker for dogs and wolves.