Actually, for me it's Giant Chasm. It has lots of powerful Pokémon there, all ranging up to level 65 (excluding Kyurem and Pokémon found using the Super Rod), and the Audinos go up to level 55.
For EV training, though, the best place for Attack and Defense would be Relic Castle by a long shot; for Speed, just fight some Swannas on Marvelous Bridge; for Special Defense, just take a ride along Route 17. There's still no decent area for training for Special Attack though — Celestial Tower's Pokémon all give away just one Special Attack EV apiece while the aforementioned areas have lots of Pokémon that dish out two or three.