Iris in Black 2. She was a nightmare. This was during the rematch - she wasn't too bad for the initial match. Her first four Pokemon didn't put up a fight, but her Archeops was a killer with its Stone Edge and Acrobatics. Then, when I finally beat it, she pulled out the big gun: Haxorus. That Haxorus was the hardest Pokemon I ever faced, what with Dragon Dance and Outrage. It took me 15 Revival Herbs. Luckily my Chandelure's Flame Body was able to burn it. I kept spamming Revival Herbs until the burn caused Haxorus to faint. Thank God Iris used up her Full Restores before Haxorus. (At least this was normal mode - if it were Challenge Mode, Iris might have used a Full Restore.)
Nishino in Castelia City was also hard. His Lickilicky had four uber moves and they kept fainting my Pokemon.
Cheren was pretty hard for a first Gym Leader. Riolu had no good Fighting moves, and Cheren's Lillipup kept using Work Up.