Well, it alldepends on how you want the difficulty. Bulbasaur for a very easy beginning of the game (only hard gyms are Blaine and Sabrina), Squirtle for moderate (hard gyms are Erika and Lt. Surge), and Charmander for hard (hard gyms are Brock, Misty, and Giovanni). But in the end, choose Charmander if you want a solid Fire type, because it will be a while before you get one other that Flareon, which is also a nice choice. Get Bulbasaur of you want an easy time with the gyms, backed up with a decent water pokemon, which are very common, or you could get Vaporeon, which is also nice. Finally is Squirtle, which is my personal favorite. Squirtle has quite an easy time with the first gym, the second gym is moderately hard, and no one would dare to use Squirtle against Lt.Surge, unless they had a decent Geodude/Graveler or Diglett in their team. Also, Erika's gym might be a challenge as well, but a strong Pidgeotto/Fearow and Flareon or Vulpix/Growlithe is all you need. Just choose who you like and enjoy the game.