I remember an interview I heard on EverFree Radio with Mitch Larson aka M.A. Larson. He said his favorite character to write for was Rarity, but the hardest one is Fluttershy.
For me writing fanfics on here, the hardest character to write for is Pinkie Pie. I try to think of something funny for Pinkie to do, but can't seem to warp my head around it.