Pikachu at earliest point.
Pikachu not charmander!! Not squirtle!! Not bulbasaur!! PIKACHU!!
Okay. In Pokemon leaf green you do not get Pikachu as a starter pokemon. However he is avalible before the first gym leader - somthing that many don't know. You can get him so early you could treat him like your starter pokemon.
Pikachu is acually avalible in veridian forest but he is rare. Just walk around in the first lot of grass and you will find him sooner or later.
In Pokemon yellow however you can get pikachu as a starter Pokemon! It also follows you around - somthing added into some of the newer games.
Added by: red 12 12 Sep 2nd 2011, ID#13551