The best
Arcanine
Arcanine has excellent Speed and Attack, which are wonderful attributes in-game. A potential moveset I would recommend:
Arcanine @ Charcoal
Ability: Intimidate
- Flamethrower / Flare Blitz
- Wild Charge
- Extreme Speed
- Crunch
Talonflame
Talonflame is incredibly fast, and while its Attack and Special Attack aren't anything spectacular, it does learn some pretty solid moves. It can be a bit recoil-heavy, however. A potential moveset:
Talonflame @ Sharp Beak / Charcoal / anything
Ability: Gale Wings if you can get it, Flame Body if you can't
- Quick Attack
- Flare Blitz
- Brave Bird
- Steel Wing
Alolan Marowak
While slow, Marowak can be a solid team member. It can learn powerful attacks, and is good even without Rock Head.
Marowak-Alola @ anything
Ability: Rock Head / Cursed Body
- Flare Blitz
- Stone Edge
- Earthquake
- Shadow Bone
Very good
Magmortar
Magmortar has an excellent Special Attack stat, and it learns a variety of good moves. It does unfortunately require trading, but if you can trade, it's a great asset.
Magmortar @ Charcoal
Ability: Flame Body
- Flamethrower / Fire Blast
- Thunderbolt
- Psychic
- Focus Blast / Earthquake
Volcarona
Volcarona would easily be one of the best Fire types if it wasn't so difficult to find. It starts off slowly, and evolves very late, but if you're willing to put in the effort (and lug around a Larvesta for most of the game), then Volcarona is well worth it.
Volcarona @ Charcoal / Silver Powder
Ability: Flame Body
- Flamethrower / Fire Blast
- Bug Buzz / Signal Beam
- Psychic
- Giga Drain / Hurricane
Houndoom
Houndoom is a little tough to get, but it's pretty solid. It evolves early, has good Speed and Special Defense, and has an array of solid moves it can use.
Houndoom @ Charcoal / Black Glasses
Ability: Flash Fire
- Flamethrower / Fire Blast
- Dark Pulse
- Sludge Bomb
- Hyper Voice
Alright
Flareon
Flareon is iffy. It doesn't have a whole lot of options. Its movepool is shallow, and it's outclassed -- both by other Fire types, and by the other Eeveelutions.
Flareon @ Charcoal
Ability: Flash Fire
- Flame Charge / Flare Blitz
- Quick Attack
- Return
- Superpower
Turtonator
It has amazing Defense, but it's slow, and its Special Attack isn't the greatest. It may also be a bit tough to get, but it can serve well.
Turtonator @ Charcoal / Dragon Fang
Ability: Shell Armor
- Flamethrower / Fire Blast
- Dragon Pulse
- Flash Cannon
- Hyper Voice / Focus Blast
Pyroar
Pyroar has an awful movepool and three bad Abilities, but it is fast and it sports a solid Special Attack, so it works.
Pyroar @ Silk Scarf / Charcoal
Ability: Unnerve
- Flamethrower
- Dark Pulse
- Hyper Voice
- Will-O-Wisp
Don't use
Oricorio doesn't get any reliable Fire STAB except Revelation Dance. It has a low BST as well, making it pretty useless. Torkoal is way too slow for in-game. It works, but if you're looking for a slow special attacker, Turtonator is better. Blacephalon is very good, but you get it post-game, making it useless in a regular playthrough.
Note I didn't include Island Scan or Ultra Wormhole Pokemon, as those take extra time to obtain and aren't necessarily good. The only non-legendary ones that would be worth it are Charizard, Blaziken, Infernape, and Chandelure.
Hope I helped!
