I'd definitely choose Bulbasaur, without hesitation.
First of all, I feel that Bulbasaur, Ivysaur, and Venosaur are extremely underrated because of their glaringly obvious advantages to the first three Gyms. Yet, they are still very powerful, and when I played, just to prove a point to my friend I just wouldn't not evolve Ivysaur until his Charmeleon evolved, and you know what? My team won.
Also, I feel that it can learn great moves. It's special defense is rather good, and can withstand an Ice Beam or even a STABed up Flamethrower, and still deal some damage. Also, it can learn Earthquake, to get rid of those nasty Fire types. It's only real weakness would be Flying, because it doesn't work on Special Attack.
Overall, I feel that Bulbasaur is the best starter, by far.