Charizard; has a great attack, excellent Sp. Attack, and great Speed. It is dual type, Fire/Flying. This makes it very resistant to a lot of things, but against Rock it will surely die. It's natural moveset is a bit bland, and the TMs you can teach it aren't all that great.
Blastoise; great defense, nice attack, excellent Sp. Defense, and a fair Sp. Attack. It's natural moveset is nice, and it's TMs are very good. It's resistances aren't all that great, but it's weaknesses are a less than Charizards.
Venasaur; worst starter for sure! Horrible stats, and a terrible move set. It's resistances are horrible, and has a quad-weakness.
Verdict; Blastoise followed by Charizard