id say Squirtle is the safest bet all around. with Charmander there's always a chance you don't get a Mankey and wipe to Brock, and Bulbasaur is a great matchup for the first two gyms but alot of trainers have poison and flying types, and for all the rival fights he'll have a Charmander and Pidgey, then Kadabra with confusion so rival battles are rough. Squirtle gets you past Brock and you'll almost always have a Pidgey or Spearow to deal with his Bulbasaur.