That really depends on the game. Though for the ones that don't have a vs. Seeker, I tend to train by continuously battling the Elite Four. I then rearrange my EVs (Effort Values) to best suit the stats that would be the most beneficial to my pokemon.
The only good place to train is the elite four unfortunately.
A good place to train is the moonglow quest located in amityvale, Blacksmith dude
A good place to train glaceon is route 217 and route 216. Hope this Helps:)
EVERYWHERE!
I've heard the conveyor belt in Geno Dome is a good place to train
actually the best place to train is in the wastlands where buu was when i trained there i 200 quick
Anywere but mostly in caves
a place with strong Pokemon
victory road is a pretty good place
In a place that many people go/ use. Eg train stations
indias a good place to try
the elite 4