Saying Bug would be the easiest answer for me, but why not change it up and be a Fire-Type Gym Leader?
Here's what my team would be:
Lead - Torkoal | Sets up harsh sunlight with Drought, can then use Yawn, Lava Plume, Stealth Rock and Rapid Spin to stand its ground.
2 - Hisuian Arcanine | IF I could get a Hisuian Growlithe egg from Perrin, I could use it to deal bonkers damage with Choice Band and Rock Head.
3 - Alolan Marowak | With the Thick Club you can also get insane damage output after using Swords Dance, and with Tailwind from Talonflame Marowak can (briefly) sweep.
4 - Talonflame | Gale Wings + Brave Bird = damage. The bird gets ground immunity, Tailwind, Defog, AND Taunt. There are many moves to make use of!
5 - Mega(?) Houndoom | From here I'm really grasping for straws not labelled as a Starter or Legendary, so under a Solar Power + Sun + STAB Overheat the dog can melt even the most resistant of steel.
My Ace - Volcarona | I would NEVER ignore my second favorite bug from this, it gets Quiver Dance after all!