Whitney. I never played G/S (my 1st game was Ruby), but she was such a pain in HG/SS. Seriously. I did beat her on the 1st try, but I wasted about 35 super potions, and she took out all but one Pokemon with that stupid freaking Miltank.
Also, I found Brawly difficult in Ruby. Makuhita destroyed me the 1st two times I fought, and I got really frustrated (I was 8). But then I literally spent about a solid week training my Grovyle, and unknowingly got him up to level 29. I was still nervous about losing, but then Leaf Blade knocked both Pokemon out in one hit. Yeah, so not really difficult anymore.