It's something of a tie between Hoenn and Johto. Hoenn had amazingly diverse Gym Leaders, and each of their Gyms were difficult and challenging. Johto's Gym Leaders also had noticeable personalities, and they were probably the hardest of the three.
In the end, I'd have to pick Hoenn. I voted Johto without thinking, though. So it should be Hoenn D=
Kanto? Lol. Yhalothar most pathetic Gym Leaders in the history of Pokémon =D Woefully easy to beat, laughably easy to get through their Gyms, and very few of them even had personalities. Seriously sad.
~*CB*~
The 8th Champion