I never got far in any game, so I only battled Cynthia and Steven.
Steven weren't really hard because he focus on two or one types and I just brought my Fire and Fighting types Pokemon. Cynthia was harder because she had many different types, I transfered a few of my GBA Pokemon to the game and I still lost. On the 5th try,
I finally won with my Walrein using Blizzard on her Garchomp! So I say that Cynthia is a much stronger opponent than Steven. Maybe the strongest.