I'm going to pay no attention to the anime for this, as this is asking us who are favourite gym leaders are game-wise.
I'm apathetic towards the triplets; although they were a really interesting idea, none of them have a really distinct personality.
Lenora is good, especially considering her toughness. Same goes for Burgh, although they could have done so much more with them.
Elesa owning Bianca's dad was awesome, and she has a unique, though appealing design, and can be a challenging boss because of her Emongas.
Clay has a very unique, although irritable, personality, and that produced a couple of interesting moments. Unfortunately, like Burgh, they didn't do enough with him.
Skyla is okay. I really don't find her that appealing anywhere, but there's nothing to dislike about her, either. Except that last time I played through White, I absolutely annihilated her.
Brycen... well I really like his badass sensei persona, the man of few words who can say so much. I really seem to think he stands out as a unique leader.
Iris is really fun in the game, especially her dialogue about being Bianca's bodyguard and her peppiness.
Drayden... idk.
So, I guess that means Elesa, Brycen, and Iris for me.