My six would be:
Pikachu
Charizard
Snorlax
Sceptile
Krookodile
Bulbasaur
*Note - I'm putting Bulbasaur in for support purposes, which I think will be key in beating a team like Cynthia's since I think only three, maybe four Pokémon of Ash's: Charizard, Snorlax, and Sceptile for certain, with Pikachu as a possibility (he's too inconsistent to reliably have a chance of beating one of Cynthia's Pokémon), will defeat any of Cynthia's Pokémon one-one-one. With help from Sleep Powder or Leech Seed, though, that could allow them to last longer and tag team one of the stragglers and/or give Krookodile the ability to win, assuming he doesn't gain the power to do so upon evolution.