Shoal Cave, north of Mossdeep city.
At the Pokemon league.
find a place that has wild Pokemon the same level as it.
Pokemon league :P
elite 4
route 205 for Pokemon diamond, pearl and platinum. the trainers have Pokemon from level 10 to level 17
Well that is a hard question. It all depends on the level of your pokemon. To train level 25-30 pokemon the best place is Desert Resort. Another place that is good is Cold Storage.
The best place to train Pokemon level 40 and above is at Mt. Silver. Since you cannot gain experience through multiplayer battling, the highest leveled place is Mt. Silver. Pokemon average level 40-49 in Mt. Silver, so it is a good place to train.
Route 201 or 202 or 203
the place where you can breed Pokemon next to heart-home city, that's where i go
The Pokemon league is proably the best place to train your Pokemon.If they are a low level give them an EXP share.OR you can train at Mt.silver and also if high enough battle red for a ton of xp
i trained mine at mt.cornet
Around the vicinity of celadon city.