Turtwig from the Sinnoh generation, Bulbasaur from the Original, Paras from the Original or Spiritomb from Sinnoh. I always liked the grass types (starting with grass Pokemon makes the first two gyms really easy, then it just stuck to me) and then Bug types joined my favorites shortly thereafter. Spiritomb is one of the closest things to a Legendary Pokemon without actually being a legendary. Dark and Ghost put together kill all of its weaknesses and that can pretty obviously count for a lot when what should be a weakness is either countered by the other type or completely nullified because of that type's immunity.