I think Litten is your best choice. It does well against Mallow's and Acerola's trials, plus Acerola in the Elite Four and part of the Champion's team, provided It lives that long. It isn't weak to the ever-present Fairy types like most Dark types, and good Fire types are harder to come by than good Grass and Water types. Incineroar may be slow, but it has an excellent Attack stat and decent defenses. It has an excellent movepool, learning things like Flare Blitz, Darkest Lariat, Drain Punch, Iron Head, Earthquake, and more. Fire and Fighting are both excellent offensive types, and Incineroar can learn a surplus of each. It being weak to most of the Kahunas and a few trials isn't a dealbreaker, as you can find other Pokemon to fight as well.
Any of the starters work, but Incineroar is best, due to a wide movepool, a large Attack stat, and advantages against a fair amount of major battles.
Hope I helped!