My favorite Fire Type in Alola would be Alolan Marowak! It's a Ghost/Fire Type, which is my favorite Dual Typing, It looks cool and is overall a decent Pokémon.
I have to say Slandit, I like the little lizard's design and he is very cute to start and is an interesting typing, fire poison.
Inconeroar. I love its personality and behavior plus the typing was an excellent troll on Pokémon's behalf. He looks like he could play linebacker XD
I've used Salazzle a lot in my playthrough, though it does have the issue of a x4 weakness to ground. I think I prefer Salandit's design though.
I just realized that there are so few Fire types... huh. Anyway, I would have to say Turtonator. It's design is pretty freaking cool. I don't know why though... reminds me of Gamera I guess~ PLUS IT'S A DRAGON!
Incineroar, salazzle, and turtonator all the way. They look cool, I like turtles and this one reminds me of one of my favorites (aligator snapping turtle), the ability to poison anything and the interesting design, and the fact that we have a trolling luchador cat is awesome! Gamefreak did a good job with these.
I'd have to say the salandit line, I really like that little poison spitting fire lizard, I enjoyed using one quite a bit.
The Salandit line was probably my favorite. I really liked the designs GameFreak fan up with, and its ability is pretty good too. I would've chosen Vulpix, but you know... not a Fire type in the game...
Oricorio because it looks like a salsa dancer xD I think it's also pretty neat how it changes type and form.
Inciniroar is my fav fire type in Alola. Alolan Marowak is cool too and would be my fav fire type if it wasn't the bane of my existence in competitive battles. It's more my fault for making my team as it is but still.
Salazzale is my favorite fire type in Alola. I prefer it over Salandit mainly because it's bipedal, which I think suits it better than quadrupedal. While its x4 weakness to Ground is a shame, it still rocks in my eyes.
I really like Alolan Marowak's design in the games, but I also really like Litten because it's a fire cat! Both are really cool though, and I appreciate them both.
I'm torn between Alolan Marawak and Salandit. I like Alolan Marawak because, well, it's a ghost/fire type and it has a cool design, but Salandit is equally as cool (hot? lol)
Overall I'd say Alolan Marowak is my favorite Gen VII Fire-type Pokémon. But I find it weird that there isn't an Alolan Cubone. Cubone goes from pure Ground-type to Fire/Ghost which is one of the only cases of a Pokémon completely changing its type upon evolving and it just seems weird.
My favorite Alolan Fire-type would have to be Incineroar. A bit of an unpopular opinion, I'm sure, but it really grew on me a while after its reveal. My second favorite by far is Salazzle, though.
Sadly there weren't a lot of new fire types in Alola, but I really like the Salazzle line, the fire Oricorio, and Alolan Marowak.