I voted Kanto because of Blue in SS. Took me 2 times to beat it. Which I usually get all of them in 1 shot.
Johto.. You know, I gotta say, I ace'd most of them. Whitney's miltank was no problem >:D
Hoenn: I dont really remember anything from it, Except the one in that city before that small city was hard. Cant remember names or types because I was like 4.
Sinnoh: Maylene for her lucario. I think it usually takes me about 4 times to get it? And that ice one. Can't remember her name, Butit kept coming back on Torterra x Froslass... (Spelling?). It took me like a whole month to beat it. My torterra was at lv 58 by the end of that, Which is E4 worthy XD
Unova: Elesa takes the cake. Sooooooo.... Harddddd..... Stupid volt tackle took me down a lot ¬.¬ It was so fast!