Suppose Ash can use any of his pokemons including reserves and released pokemons and the battle against Leon is 6 vs 6? Which pokemon do you want to be at Ash's team? (Pikachu is mandatory)
What I think would be Ash's strongest team possible for that match and why:
1-Pikachu (G-Max)
2-Charizard
3-Sceptile
4-Greninja (Ash-Greninja)
5-Incineroar (Z-move) or Snorlax
6-Lucario (Mega) or Dragonite
Charizard is his OG ace and his strongest non-transformed pokemon. Sceptile has proven himself against Darkrai. Greninja's transformation makes him OP. The remaining 2 slots could go to different pokemon.
Snorlax has shown incredible endurance and is super reliable, while Incineroar was capable of defeating Kukui's as a Torracat, so we can only imagine how much stronger he is now.
Lastly I think they're hyping up his Lucario to be pretty OP, and I can see Ash getting a Mega stone for him. If not, Dragonite has already proven to be very strong against Korrina and can only get stronger by the end of the series.
What I think he'll end up using:
1-Pikachu
2-Lucario
3-Dragonite
4-Charizard
5-Greninja
6-The last Journey's capture or Sceptile/Incineroar/Melmetal (G-Max)/Lycanroc