I would have to say charizard would be Ash's best and strongest Fire type because think about it, in addition to all the pokemon Charizard had fought against he also went training in the valley of charizards to get super stronger! he would beat any of Ash's other fire types hands down.
I would like to say that Infernape is a strong contender but it would not have the defense to stand up to charizard and only IF Quilava had evolved into typhlosion then it too would have be a contender. (I still hate the episode with Cyndaquil versus Steelix. How dare the writers make a cydiquil fight a huge and evolved pokemon like steelix and not evolve itself? I call shinanagans!)