Assuming that you are talking about G/S/C, I'd have to echo that Mt. Silver is a pretty good place to train your Pokemon. Facing the Elite Four again isn't a bad idea, either. Sure, they're levels don't increase for the rematches, but Lance's Pokemon range from levels 44 to 50- roughly the same as those inside Mt. Silver (and then you have the ability to face Red again after each Pokemon League victory).