ok, Charizard, Pikachu, Sceptile, Kingler, Donphan, Snorlax
Breaking it down
Pikachu-Definately has the most potential could be the most powerful pokemon in the world if it wanted to, if ash would have trained it more in Kanto and Johto instead of spending all their time strengthening their freindship(which is good, dont get me wrong) pikachu would have no problem taking down almost any pokemon in its way
Charizard-A natural born power house. I think one of the main reasons he's so powerful is the time period where he refused to listen to ash, i think he, and ash too, came out of that two times stronger. Also, both the natural, wild training and the trainer training is very good for it and it could probably take out any Charizard in its way in a little more time in Charific valley
Sceptile- Definately not as nearly impressive as Pikachu or Charizard, but he was the power house of the battle frontier. I think Sceptile was the leader of the battle frontier and hoenn teams, as well as the stongest, besides pikachu of course. His intense serious atitude makes him show no mercy during battle. What limits and sets him apart from Pikachu and Charizard is his limited move set.
Kingler- Kingler has only a little less potential than pikachu. He came into a Indigo League match and defeats all three pokemon with relative ease, ones that probably have been trained for at least a year. it has a huge move set and a kick ass Hyper beam taht blew my mind. If Ash would've trained him half as much as he's trained any other one of his past team pokemon, he'd prolly be just as powerful as Charizard.
Snorlax- I havn't seen too many of Snorlax's battle, but the ones i have seen, Snorlax totally kicked butt. From its ice punch to its hyper beam, its a total powerhouse.
Donphan- Is amazing. Even before it evolved, it was a kick ass pokemon. Its roll out and hyper beam are awesome moves, again, another power house!