Fast Experience and Leveling!

*Ghost Type Pokemon are the easiest to train!*
Let me explain! Ghost pokemon are unaffected by the NPC Bird Trainer. The fastest way to gain experience would be to have a very high leveled pokemon (For starters, you could have someone loan you a high leveled pokemon; there are plenty of loan threads!) in the second slot of your party, have the ghost pokemon as your starter. Make sure you have Bolt Strike and/or Blizzard in your move-set. Use either move to kill the two bird pokemon; with a really high leveled pokemon; you could gain 100s of thousands exp each battle.

*For Goletts/Golurks Only* - Credit to AncientVampyr
Make sure your moves are: Blizzard, Bolt Strike, Blue Flare, {Anything}
Fight Zapmolcuno (bottom in NPC fights)
Use Blizzard on Zapdos, Bolt Strike on Moltres, and Blue Flare on Articuno.
Sometimes you will die, but if you keep trying, you will get lucky and win (I win just more than half the time), and get enormous amounts of EXP!

*Any other type of pokemon!*
A little more annoying to train non-ghost type pokemon; but it's still not hard! For this you must have only the pokemon you are training in your party. Make sure you have the the move Bolt Strike! Battle the first swimmer; Swimmer Richard, under the Intermediate section! Now strike those bolts and win the battle! Your opponent will be double your level, giving you a great amount of experience, especially when your higher leveled.

If anyone has anything else to add please post it and I'll add it :).