This is basically a toss up between Kanto and Johto. Why? Because the other 2 gens don't have Blissey or Chansey =P
Personally I pick Johto because it's got the best walls (Donphan, Skarmory, Forrestress, Blissey, Suicune) and the best attackers (Heracross and Tyranitar, Raikou). Throw in some...