I think GSC have the best level curve in the series, and that includes postgame in Kanto. Tamashii is right, you're meant to train up potentially dozen of pokemon little by little each day. The level curve accommodates this and you really get a lot more out of the time based elements if you play this way, which the game is heavily designed around. This may not be for everyone but I find it the most in-line with my own personal preferences