As nintendosmashe said, the best place I believe would be Victory Road. Find some Zubat/Golbat to raise your Pokemon's Speed Effort Points and some Geodude/Graveler to raise your Pokemon's Defense Effort Points.
try the sevii islands, you can't get to the other ones but the first 3 are pretty good, like Mt. Ember on 1st island you could train a water or ice type pretty well. and so on at the other islands or find a better place, i usually train on the sevii islands or against the elite 4 themselves. you never know you could be like me and just train against the elite 4 and i actually beat them on accident! it was pretty funny
Vs. Seeker on places... like on that route to the right of Fuschia city. Or maybe Victory Road, but I hate the Golbats and such. Yeah, go One Island, against all Rapidash and Meowth. I found a Shiny Meowth there once... but I digress. That's a good place to train.
I once heard Seven Island, the two trainers right outside the shack with the Chansey Dance man. Fairly good exp, and you can heal inside if you dance. Its a little rough, though, as each trainer has 5 pokemon lined up.
Then again, thats AFTER the Elite 4, unless you're training for the second pass at them.