each starter has its own qualities, but i would like to think of it in terms of the gyms they would be good and bad at based on type matchups:
pewter: bulba-best, squirt-best, char-bad; winner: bulba & squirt
cerulean: bulba-good, squirt-neutral, char-bad; winner: bulba
vermilion: bulba-good, squirt-bad, char-neutral; winner: bulba
celadon: bulba-neutral, squirt-bad, char-good; winner: char
fuchsia: bulba-bad, squirt-neutral, char-neutral; winner: squirt & char
saffron: bulba-bad, squirt-neutral, char-neutral; winner: squirt & char
cinnabar: bulba-bad, squirt-good, char-neutral; winner: squirt
viridian: bulba-good, squirt-best, char-worst; winner: squirt
1st half winner: bulbasaur
2nd half winner: squirtle
AND THE WINNER IS: SQUIRTLE!!!
ok, ok, whatever. :\ bulbasaur's good at the beginning, charmander's good in the middle, and squirtle's good at the end. so... take your pick.