Once my Pokémon were at Level 55-ish, I used the VS. Seeker on One Island, on Kindle Road, specifically against Crush Kin Mik and Kia, as well as Black Belts Hugh and Shea. They are right next to the hot spring room (Hugh and Shea are walking back and forth in front of it), allowing you to heal your Pokémon when needed. They give about 1000-1600 EXP per rematch. [source]
Another good place is on Cycling Road. Koffing give a good amount of EXP as well and they are plentiful there. However, there are plenty of Pokémon that know Explosion, so you might want to keep on your toes. [source]
Other options are available, as well. Some might not be available till after the E4, though (specifically those in Four Island and beyond).