I've compiled a complete list of Vs. Seekerable trainers here, including a "High-Level Training Spots" section, where I've concluded the best training spots are these two:
Ace Trainer Dennis and Ace Trainer Maya, Route 215 (just west of Veilstone City)
Bird Keeper Audrey, PKMN Ranger Ashlee and Psychic Daisy, Route 225 (just north of Fight Area)
The latter gives slightly more experience all in all but involves more trainers and Pokémon, so the first is generally better.
Hope that helps.