Ok when you think of the best a few names come to mind and i mean his actual best ones no just ones that the writers make to look good or bad or because ash is to stupid to use them.
charizard
he is the first one to fully evolve and be an actual power house (Dont say butterfree......) and he has won a battle against gary's blastoise and an articuno but was first to fall against brandon and he fell fastest and easiest in comparison so squirtle and bulbasaur plus he is toast to a rock attack
snorlax
this guy is good and by some miracle fast, he took on two of the fighting frontier brain pokemon and did well, he has good defence and attack we all know as deadly though he has never had a continuous time to shine only with a few frontier battles and league fights. he packs a mean punch
heracross
he too has a major weakness but he is tough and has been ashes last resort many times and did great against gary while he was lituraly on fire, his hand to hand combat, endurance and pure power are his best assets in my opinion and he is pretty well ashes only jhoto power pokemon yet he is sent to the lab for the whole season..... and so far he is the only one to dodge in his sleep.
quilava (not realy any bragging rights)
ok he isnt that good but has some good moves with airal ace and eruption, he is probably ashes first and topmost fire type for speed since charizard was a fighter
sceptile
ok lets look at him, he has the most bragging rights. he beat a legendary and is one of three to do so. charizard had a type advantage and pikachu had a super effective move while sceptile had it hardest with his oponent having the super effective move. plus he beat arguably the strongest legendary ash has fought seeing how it had swept whole teams by itself. it also beats the darkrai fair and sqaure on its own since all damage done by hercross is most likely healed since darkrai got half or hercrosses hp which is probably enough to fill the gap hercross made and the only damage it took which is pretty much none was from rock smash which is later healed with another dream eater once until sceptile beat it to death with ONE leaf blade, name me one other pokemon of ashes to beat any legendary with ONE attack, let alone one that sweeps whole teams. Plus if it wasnt for sceptile i guess ash would have been like all the others with a clean loss only to darkrai.
infernape
in my opinion this guy is a sorry excuse for drama. he is only tough because the writers concentrated on him so much and tried to make him look like a legend with his blaze, put him with darkrai and he will burn out. but seriosly this guy is onyl tough because the writers made him look good with his blaze since he used to belong to paul but he is nothing more than a pretty act in my opinion.
torterra
ok he is on a loosing streak since the writers were focusing so much on infernape and he basically had to lose his battles but if they got rid of suacide mountian which he keeps falling off and ash would stop thinking he is a sceptile with unmatched speed and deadly special attack he has potential though, i wish he got earthquake instead of the always backfiring rock climb. i hope ash learns or he will never survive with emboar.
gliscor
eigh cant say much here, he cant even fly properly but he does have a good giga impact strategy.
squirtle
this guy is one of ashes oldest pokemon and fared well against brandon.
bulbasaur
i think he did the best against brandon with dusclopse and solrock at bear minimun while charizard lost before winning and squirtle just beat ninjask and little old pikachu with regice.
pikachu
your kidding right? this guy's level resets all the time! he goes from beating all the gyms to beating a regice (even if he did have iron tail) to being beaten by a novice (well he did enter a league but elekid is new) elekid, an elekid! then after fighting well in the sinoh league gets beaten by a trainer and his snivy which have been training for lituraly five minutes! i dont know about you lot but i recon he is a sad excuse for some drama and a partner and not to mention ending in a TIE with a latios which fought four pokemon prior.

my vote is for sceptile, his battle with the ultra powerful darkrai who was at full health seeing as it restored from booth hits with dream eater and was still OHKOed by sceptile. plus he packs a mean punch and has the awsome speed of a good fighter. though i cant forget to mention his epic battles which in my opinion was the single best battle in the anime, the one with the guy with the venasaur in the battle frontier, i mean it was the biggest and most complex battle arena plus it was plain old awesome.

but feel free to remind me of ones i have missed

if someone votes that sorry excuse for a yellow rat i will strangle you, he looses to every new trainer he sees!