FR/LG (Never owned RB): ARGH! IT TOOK ME ABOUT 15 TRIES TO BEAT THEM.
GSCHGSS: Freaking Lance. Stop cheating. Just stop. I had so many problems, even with a Vaporeon knowing Ice moves in SS.
RSE: I don't remember it being very hard, honestly. But my Swampert was MAJORLY overlevelled by the time I took them on so...
DPPPt: Diamond was rather difficult for me, actually, since I didn't have a lot of Pokemon on my team with type advantages (Had some for Bertha and Flint, that's it). Platinum was easier as I had a Fire type, Grass type, Water type, and Dark type in my team.
BW: Easiest ever. I'm not complaining!
B2W2: Still easy, but Iris was challenging.