experience

Experience
Each time your Pokemon defeats an opponent Pokemon, it will gain some experience (EXP). EXP helps you grow levels and levels help your Pokemon evolve and learn new moves. All Pokemon need a certain amount of EXP to reach lvl. 100 and all opponent's give a certain amount of EXP.



EXP Totals
These are the six categories that each Pokemon will fall into when determining how much EXP they will need to reach lvl. 100.

Erratic - 600,000 EXP
Fast - 800,000 EXP
Medium-Fast - 1,000,000 EXP
Medium-Slow - 1,059,860 EXP
Slow - 1,250,000 EXP
Fluctuating - 1,640,000 EXP

Each of these numbers are arrived at by using specific mathematical formulas. For those of you interested in these formulas, you can check them out at Bulbapedia. For those of you that don't like math or don't care, move on.

EXP Yield
The amount of EXP that a Pokemon will give you all comes from one single formula:

EXP = ((Base EXP yield x Level of opponent) x Trainer x Wild)/7

In order to figure this out, you need to know the Base EXP that the Pokemon you're fighting gives. Base EXP is explained below. Level of the opponent is self-explanatory.
The Trainer value is either 1 (if the Pokemon's Trainer ID matches your Trainer ID) or 1.5 (if the Pokemon's Trainer ID doesn't match yours, meaning you got it through a trade).
The Wild value is either 1 (if the opponent is a wild Pokemon) or 1.5 (if the opponent is trained).

Base EXP
All Pokemon have a Base EXP given to them that tells how much EXP you'll gain when you defeat them.

The Pokemon with the lowest Base EXP yield is poor little Magikarp with only 20. The ones that have the highest Base EXP yield are Chansey and Blissey, with 255. So fighting a team of Chanseys or Blisseys will give you more EXP than fighting any other Pokemon.

Let's see how much EXP a trained Lv. 100 Blissey will give a Pokemon that you obtained through a trade.

EXP = ((255 x 100) x 1.5 x 1.5)/7 = 8,196.42857

This number rounds to 8,196. That's a lot of EXP! If you can somehow manage to fight a team of six of those Blisseys, you'll gain a whopping 49,176 EXP!