Actually, Grovyle has the lowest defense of the bunch. Ok...
Treecko: best at special attack and speed, learns leaf blade naturally (if you let it evolve), can inherit dragon claw (tm or from salamence) and crunch (salamence or seviper, who learns it earlier). Low defense/hp and so-so attack. beware of wallace's ice beaming milotic in the final gym.
good early partners: wingull, makuhita, electrike
Torchic: best at special and physical attack, if evolved learns double kick, bulk up, and sky uppercut naturally, blaziken learns EQ from a tm, and can inherit rock slide (from camerupt). Unless you're crazy enough to hold off torchic's evolving until level 49, you need to teach it flamethrower with a tm/breeding. low-ish HP and defenses.
good early partners: breloom, aron, electrike
Mudkip: best against most of the gyms, evo's are part ground, if evolved learns EQ naturally, learns ice beam via tm, the only "bad" stat is speed. Has only one weakness when evolved: grass. most say it's the best starter, but all are good. when fighting flannery use ground move mud shot, for sunny day will weaken any water move.
good early partners: breloom, taillow, electrike (i discovered you can beat the whole game using only swampert and breloom)
Hope that gives a clearish idea...