In Fire Red, there are three great spots:
1) The gap between Erika and Koga is huge!!! Luckily, the group of Bikers near the Celedon entrance/ exit to the Cycling Road (depends on which direction you're coming from) have strong enough Pokemon to train against.
2) Prior to fighting the League, that space just outside Ember Spa on One Island is ideal. Powerful trainers to fight against and a healing spot nearby.
3) Post Game, the best place to train (not just in FR/LG but in all of 3rd Gen) would be on Seven Island, in Seavault Canyon, just before and right by the Chansey House. Trainers with ridiculously high leveled Pokemon and a healing spot within a short distance.
Vs Seeker FTW!!!!
Emerald on the other hand was a pain. No great training spots prior to fighting the League. Your only option is an extended stay near the end of Victory Road (I despise Hariyama, Lairon, Whirlwind and Knock off). Post Game, the only option you have is to repeatedly challenge the League.