Training place?

I always train my Pokemon in Mewtwo's cave the Pokemon there are strong mid-level Pokemon they range from level 40-65. The only other quick method of training Pokemon quiker is by battling in the Elite Four. (WARNING RARE CANDIES) There are harsh consequenses for training any Pokemon with rare candies. He's what happened to me. I had a mewtwo at level 75 despite the fact that it's already powerful I wanted to train it quicker just to expeirence how powerful he was unfortunetly this wasn't the case. After investegating the situation I finally found the answer and the harsh penalty that came with it witch was his stats and they look like this.
Hp: 348
Attack: 251
Defence: 201
Spl Attack: 328
Spl Defence: 208
Speed: 275
Those stats are a result of a Mewtwo being trained with rare candies to level 100. His fully trained stats are
HP: 415
Attack: 318
Defence: 278
Spl Attack: 406
Spl Defence: 278
Speed: 358
I apoligize for this answer being so long I'm just trying to get the point across from witch rare candies are no good for your pokemon.

I would say to train in seavault canyon using the VS Seeker. To get some rare candies (not alot, but some) try to carry a meowth in party and start wandering around the islands.

Go too One Island and right before the spa you use the VS Seeker right where the rock is and 3 people will hear battle there when your done go into the spa and do it again there you go

If you have Pokemon Emerald here is a really simple, but agaisnt the rules method as it is using a glitch. First of all, to explain said glitch. This glitch basically lets you do anything you like with your party, money and items/bag, which then saves properly, but the glitch also reverts the PC back to its savestate before you began the glitch. To do the glitch. Travel to the Battle Frontier and enter the battle tower, move to the PC. Set up your PC with all your items handed to pokemon, or at least your best ones that you want to duplicate. Next, save your game, from this point on, try not to put anything else in the PC as if it was not there already, it will bee lost. Gone. Forever. Capeshe? Next, go into the 'Move Items' mode and take every item rom every pokemon, then take out 5 pokemon you wish to duplicate, don't store the other one in your party or you will lose it. Now, walk over to the LINK MULTI lady, she should be the one closest to you, go through her dialog, accpt save, rahdy rah, but when the dialog dissapears and it would appear that you can walk away, you know your almost there. the next thing you know thouhg, the dialog reapears and the game begins to save, before it finishes, quickly restart you GameBoy/DS. When you enter the game again, you should be in front of the Link Multi lady, if you are, go check your PC, happy? i bet. If the game says "Save is corrupted, reverted to last save" or something similiar, you did it wrong :P Enjoy.

Yes you are right but if it's not ev trained then the rare candies would effect the stats if your using gameshark codes you can use rare candies to take it to level 100 and it will be at it's full potential. Also the fully trained stats that I previously shown are it's max stats and it's not possible to archieve this unless you're using a cheat device but you can get reasonbly close to those values.

If your Pokemon are lv 20-30 the best training I would say is Vermilion City go right and up from the poke center us the VS Seeker and fight 3 trainers for about 600 xp per one