^^ You can say that, but you would be wrong. Swampert is way better than the others, with Empoleon being better than the rest but still not too great. Blastoise is easily the worst of the five. But... I do love Squirtle.
Their original ability is what i voted on, since their peak is always changing and will keep changing.
Currently totodile is probably best, but at the time it was released, mudkip was best. Grass was a fairly rare and unthreatening concept back then, so in addition to it's inherent beefiness, it had some ridiculous staying power.