>>22349667Gameplay wise? From worst to best.
Wallace = Steven > Diantha > Iris > Alder > Lance > Blue > N/Ghetsis (N is a unofficial champ and Ghetsis serves the same role) > Cynthia.
Half the problem is monotype champions are completely stupid. Steven can literally be soloed with a Blaziken if you started with one. Wallace is equally bad as whilst he has a better balance, half his teams are the worst type of shitmon who barely threaten.
Diantha is equally guilty, her fossils don't even have their hidden abilities. She uses a ghost which has no immediately threatening moves with a retarded AI (Well Pokemon AI is horrible in general). One of the worst dragons ever in existance and a Mega-Garde without even it's strongest move in a gen where Aegislash was introduced.
Iris and Alder suffer from the monotype problem as well but less so in Alder's case at least he had a Outrage Druddigon to mix things up. Whilst Iris has a lot of strong mons but absolutely no type synergy to it so can be swept by a single type.
Beyond those lot the remainding Champions are pretty good to fight in their respective games.