Y'know, after rewatching many of the series, I have changed my opinion on the subject by a little. So, in terms of power, it's undisputed that Pikachu is the strongest ace by a lot, not only is he the one with the most experience and closest to Ash, but also has fought the most foes out of them all, so it's no surprise that the yellow mouse is an absolute powerhouse and that if it was to evolve, it would be a million times stronger than them due to all of its previous training combined with the massive boost from Pikachu to Raichu, which would require the others to catch up to it.
In terms of story, it has to be Infernape for sure, they took a simple ability combined with a nicely-designed starter and turned it into an excellent character that could give goosebumps to everyone watching, from a Chimchar with extremely high potential that was mistreated and traumatized from its abuse training to an absolute powerhouse that is among Ash's top 5 strongest Pokemon of all time and swept through half of Paul's team along with tutoring Champion-Level Pokemon like they are some newbies, goes to say a lot about his massive power and potential. Every moment with Infernape in DP felt great to me, from him hugging Ash as a little Chimchar while crying, which goes to show that Ash doesn't need to act extremely serious to be considered mature, and that maturity comes with responsibility in your acts and the ones around you.
In terms of iconicity, it's for sure Charizard, the anime turned it from "that cool dragon starter" to the literal most iconic Pokemon of all time and a practical mascot for the franchise in everything but the title. Of course, Infernape and other Pokemon have improved upon the basic concept of Charizard's arc with the years of more experience of writing the writers acquired, but that still doesn't change how much impact and weight this thing had whenever it was sent in OS-AG, because you never could know what it was going to do, other than the fact that it was going to be something memorable. From causing Ash to lose the Kanto league the way he did to beat Gary's Blastoise to crippling Blaziken enough for his opponent to lose the next match, and to peak in his showings by beating a legendary in a straight 1v1 without any help, Charizard was just great and I hope that he gets another opportunity to show up in the future because knowing his character, he for sure continued training and getting stronger alongside the likes of Infernape.
Now in terms of my personal favorite, it's for sure Sceptile. I have been underestimating this dude for years now, in terms of power and personality, but now that I rewatched through a lot of AG, I have fully understood just how badass this ace is. For one, he didn't depend on anything to become strong, other than his own relationship with Ash, no special training in the Sceptilic Valley, no overpowered Overgrow, no Bond Phenomenon, no Mega Evolution, no Aura Powers, nothing, just his bond with Ash that allowed him to get to the next levels, and for this I consider him to be Ash's most natural and deservedly powerful ace in terms of development and power acquired through a series. Then you amount this into the fact that Sceptile was also realistic, it didn't have a flawless W/L record of steamrolling through everything and being unbeaten at an early stage, it started out weak and inexperienced, living under Swellow's shadows, but then grew up into the strongest member of the Hoenn squad, and easily one of Ash's all-time strongest Pokemon, by learning from the multiple beatings it took.
And for all these reasons, my favorite episode involving Sceptile, and easily in my top 3 Pokeanime episodes, is Cutting The Ties That Bind, that episode just had everything in it, from Sceptile and Ash showing just how much they care about each other, to the core aspect of the Pokemon franchise and what made it unique in the first place, and then concluding with Sceptile's depression arc ending in an excellent note of saving a wounded Ash from Team Rocket before beating two Pokemon from Spencer in the Battle Frontier. The whole thing just felt really great to watch, and this combined with the previously mentioned aspects of Sceptile made me like him much more than Infernape, Charizard, Pikachu, Greninja, and others.
