Wherever you can find the highest-level wild Pokémon is usually the best place to train for EXP (if that's what you are asking for). However, you can also call the trainers on your Pokégear to fight their Pokémon at higher levels, which gives you more EXP per Pokémon than against wild PKMN.
Before the Elite 4, the best places would be Route 45 or Victory Road. Afterward, once you have access to Mt. Silver, that's the place to train for EXP.
If however you are asking for EV training, then I'd defer to any other answers people give.