Depending on what level, I can suggest Johto Route 47 for Level 35-40+.
^ See that tiny patch of grass at the top of the first waterfall? There are Miltanks there at L35, and they're a pain to take down but they give 1000 exp. each.
There's also a lot of Ditto, and if you let them transform into whatever you are, you'll get ~800 exp. for most fully evolved Pokemon.