Well it's pretty hard to grind in GSC, but if you're going against red or if you're just grinding your pokemon. It would have to be Mt. Silver, or the Victory Road. It's up to you.
Be more specific on naming a thread, I have no idea who you're grinding for.
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).