That's entirely up to you really but personally I'd suggest Arcanine since is usually a solid fire type to use but since you have Ultra Sun you actually have a few more options for fire types doing regular playthrough with the Houndour line and Turtonator.
Using Alolan Marowak is also a very viable option. When you do the Fire trial, have a Pokémon with thief steal the Totem Pokémon's item. Collect enough stickers after that, and you get a Totem Marowak that comes at level 25, which means you get Shadow Bone. Houndour and Turtonator, as Hraesvelgr suggested, are also great picks. What kind of fire type are you looking for? A physical or special attacker?
Alolan Marowak is also a really great option yes, is typing makes it pretty useful and is no weakling either.
Salazzle might be a good one, but they are very much the glass cannon. So they will require some extra care.
For main game, houndoom or alowak works great. For post game, UB Burst is a top contender with it's great Special attack and decent speed.
Good call! Definitely make sure you use Thief on the Fire Totem Pokémon - you'll get a good item for your Marowak. I'd suggest using the totem one as well, no electric immunity but you can spam recoil moves and learn Shadow Bone.
