Bulbasaur for obvious reasons, and 4 of his flying types
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If we're talking about ranking their stats/hypothetical levels, Charizard, Sceptile and Greninja are definitely up there. Charizard was able to defeat a Legendary such as Articuno as well as having beat Gary's Blastoise beforehand despite the type disadvantage, as well as consistently fight against various Dragonite such as Drake's or Iris'. If factoring in in-game logic with levels (which aren't entirely removed from the anime when the 9th episode of the anime as well as the one on the SS Anne where Ash temporarily trades his Butterfree for a Raticate alludes to them) would place a very high minimum on Sceptile when it can only learn Leaf Storm at Level 67, and its feats at the Sinnoh League definitely made it one of Ash's most impressive Pokemon with how it defeated Darkrai. Greninja is an obvious choice with it having what's essentially a pseudo-Mega Evolution that allowed it to fight evenly with Diantha's Mega Gardevoir (which is at level 68 in the games) as well as consistently compete with Alain's Mega Charizard X that had already avoided being oneshotted by Primal Kyogre before Alain met Ash and helped bring him further than in any past league. If I were to rank them, I'd probably say Greninja > Sceptile > Charizard
For the next two, at least as far as levels go, I'd probably say Snorlax and Infernape take the remaining spots, which is #4 or #5 being pretty interchangeable. Stomping two of Gary's best Pokemon is no small feat and being brought in for his Sinnoh League matches gives it some credit. Infernape was consistently treat as a powerhouse during the journey with Sinnoh when it came to its growth as well as how Blaze was treat as a game changer, particularly with how it took out half of Paul's team in the Sinnoh League.
Pikachu would definitely be included (likely taking Charizard out of the top 3) if using its peak self that beat a Regice despite most of its attacks being resisted or stalemating Tobias' Latios, though Pikachu's capabilities are obviously too wildly inconsistent to add it.