Originally Posted by Charizard
Well, Squirtle is the easiest to get since you just need to beat Lt. Surge.
And the hardest is Bulbasaur. I never knew he exact details and requirements on how to get it but I always got it with pure luck. :p
Actually Charmander's the easiest to get since you just need to talk to the kid (and getting past Nugget Bridge isn't hard).