Here is an analysis of all 4 starters...
-Bulbasaur: Maybe it is just me, but i think grass type pokemon
in general are nothing special in Gen1. Mega Drain is overrated
(it inflicts little damage and offers even less healing). Solarbeam
requires two turns to activate. Special conditions' moves (sleep
powder etc) have low accuracy. Even if you want to use Bulbasaur
for physical moves (Body Slam, Double Edge etc), there are better
candidates for that job. All in all, a mediocre choice.
-Squirtle: The best choice by far. It can defeat even its weaknesses
(grass pokemon with Ice Beam, electric pokemon with Earthquake)
and its stats are exceptionally balanced.
-Charmander: Ignoring anime, game wise, its final evolution,
Charizard, is not as great as its appearance makes it look like. Its
fire/flying combo type renders it doubly weak against rock type. It
is also weak against ice type in Gen1, even though it logically
shouldn't be. Just a satisfactory choice.
-Pikachu: This one can go to hell. Awfully low ATK, DEF, HP, and
SPECIAL (which is a number both SPECIAL ATK and SPECIAL DEF
share in Gen1). It could improve its stats somehow if it evolved
into Raichu, but it just won't accept. If you play Yellow, let that shit
rot in a box.