V6 Valerie and Narcissa for me.
Valerie : We all know why she is so OP...Her field got some of the best effect "Grounded non-water types' Speed is halved, except for Swift Swimmers",all my pokemon got their speed halved except Staraptor and Crauwdaunt.She broke all of my strategies one by one,at the end i needed to rely on Sword Dances and hope she doesn't land a miraculous crit.At least she can't do anything to some of those pokemon (Quagsire,Ludicolo,Helioptile,Sunflora,Vileplume...) but if you don't have any of those useful mon...You're going to have a hard time.
Narcissa : I don't remember if she still have her Dusclops in V6 but the one in V5 was a nightmare...I think it got replaced by Spiritomb but he's still as strong as it's predecessor...Her team is pretty strong but still very frail,that's when her "Special move" become handy since there's a chance to raise the user's defense.Once again if you have the right pokemon to counter her team you'll be fine.thanks torkoal.
I'm waiting for the Dragon type gym leader...