A good place train is the route next to Undella town. You can put exp. share on a weaker Pokemon let a stronger Pokemon do all the work and you will level up quickly because the Pokemon there are usually about level 50-60.
Another way is by using the daycare to level up fast by just using your bike or running to level up.
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The best place is probably in victory road or the route before it because of the high level Pokemon and if your asking there is no v.s seeker in Pokemon black an white
Mt. Silver or the Pokemon league
probly at the pokemon leuge
Some people will argue with me but i think the best place to train is at the Pokemon lugue you can battle them over and over again
victory road is a pretty good place