Easy. Mankey, nidoran or butterfree.
Mankey for its high attack and low kick (takes out heavy rock pokemon quite quickly) or if I wasn't in the mood for raising a mankey, nidoran could take quite a few hits while whittling down the rock pokemon's health with double kick. Butterfree with its decent special and with the rock types having not so good special, I could take out Brock's entire team with confusion alone so long as butterfree was at least level 12.
SasakiThePikachu, mankey are obtainable quite close to the start of the game, even before you reach Pewter. Try going left of Viridian City as though you were going to Indigo Plateu but don't go quite that far, the patch of grass where you find nidoran and spearow also has quite a lot of mankey.
Mankey or I'd trade over a Squirtle.