Pokemon Gold/Silver:
Several nice places to train your Pokemon are
the grassy area below Ecruteck City and above Goldenrod City. There you can find Stantler that can give you high EXP points.
the grassy area to the left of Ecruteck City. There you can find Raticate, Tauros, and Miltank that will give you high EXP points
the bug catching competition in the park above Goldenrod City. There you will find level 8-20 Pokemon that will give you higher EXP points
trainers registered on your cellphone. Trainers usually have evolved Pokemon that will give you higher EXP points
the Victory Road.
The Elite Four. This choice is obvious after having level 40 and above Pokemon
For low level Pokemon, headbutt the trees and a weak level 10 Pokemon will give you enough EXP points to bring your Pokemon to the next level.
Pokemon Ruby/Sapphire/Emerald: Several nice places to train your Pokemon are
The Jagged Pass. There you will find Numel and Spoink that will give your Pokemon sufficient EXP points. This is also Drakyria's(or Lilakyria's --manjyomejunthunder) favorite training place.
For emerald only, the mirage tower. This is the only place in the desert where you will save time by not having the sandstorm rage on and on.
The Elite Four. We recommend your Pokemon to be at least level 50 to have a quicker time training.
the Whismur cave. Whismur cave(the cave with many Whismurs) give move EXP points than the area outside Whismur cave.
Pokemon FireRed/Leafgreen: Several nice places to train your Pokemon are
Using VS seeker on the route above Vermillion City. The person with the Butterfree gives you ~600 EXP points every time!
Using VS seeker on the route to the right of Pewter City after beating the Elite Four. The first lass you see will have level 50 Pokemon with her after about 5 times of battling her.
Using VS seeker on the Cycling Road. The last biker you see will give you the most EXP points of all of them.
the Diglett Tunnel. Dugtrio are rare but will give you ~600 EXP points every time.
The Elite Four. The Elite Four level up a bit the second time so plan accordingly so you won't waste your time restoring your Pokemon and losing to them.
Pokemon XD:
Several nice places to train your Pokemon are