Whitney was a piece of cake because I went at her with female Pokemon, and only my Quilava was the only male Pokemon on my team at that time. :D
The hardest Gym for me was Clair. Not battling her, but navigating through the Gym. Not to mention my Kingdra got out sped by her Kingdra and fainted from one Dragon Pulse and I had to set up a pick so I could revive it and take it down. Not to mention, she acts like a sore loser when she lost. At least Whitney gave me my Badge after she stopped crying, but Clair, like the sore loser she is, said 'No, you don't get this Badge, but I'm a stuck-up sore loser.'
That's my blunt, straight-forward opinion.