Well, let’s look at all the gyms.
First gym, Fenniken is Super Effective. Froakie is nuetral. Chespin is weak to bug, but it gets rollout, and I took out the entire gym spamming rollout. Second gym, Braixen is weak to it, and Quilladin and Frogadier are super effective. Third gym:All nuetral, but Braixen has Psybeam. Fourth gym, Braixen/Delphox is super effective, Frogadier/Greninja is weak to, and Quilladin/Chesnaught resist. Fifth gym, Delphox is nuetral, Greninja is weak to, and Chesnaught resists. Sixth gym, Delphox is nuetral, and Greninja and Chesnaught are weak to. Seventh gym, Delphox resists, Greninja is immune, and Chesnaught is weak to. Final gym, Delphox is super effective, Greninja is nuetral, and Chesnaught is weak to but also super effective.
Rival’s team depends on which starter you choose, but they always have Absol, Meowstic, and the starter you are weak to. Chesnaught and Greninja are better for fighting the rival.
Chesnaught has an advantage over steel and water, Delphox beats steel and resists fire, and Greninja beats fire and resists water.
Chesnaught does good against Gourgeist and the two fossil Pokémon. Delphox does good against Hawlucha, Gourgeist, and Gardevoir. Greninja does good against Aurorus and Gourgeist.
So, in the end, we have:
Gyms:They all do pretty good, but Chesnaught seems to be good against the most.
Rival:Greninja is probably the best here, but Chesnaught does fine
Elite 4:Chesnaught is the best here, no doubt about it
Champion:Chesnaught and Delphox are probably better than Greninja here.
In the end, Chesnaught is probably your best bet.