Fantina was hard for me, took me at least two tries to take her down, I got all my pokemon to level 35 (For Murkrow with Faint attack and Luxray with crunch) and then she was a breeze. All the other gym leaders were too easy, i'm currently training for Volkner.
Actually, for me whitney was easy, thanks to my Flaffy paralyzing that Miltank.
Morty was harder IMO. Dang status problems.