not counting his kanto crew, these are probably his best:
sceptile swellow infernape krookodile
i didnt list heracross because although heracross was the boss in the johto region, ash hadnt used him again for a long time and also heracross is kinda lazy and just likes to drink sap all day
Heracross wasn't boss in Johto. Heracross won all of three fights in Johto:
Rochelle's (weak and inexperienced) Donphan Shingo's Scizor(Finish Pinsir off with False Swipe!). Gary's Magmar.
All but the first, it barely won.
It lost twice, as well. Rochelle's (strong and most experienced) Donphan, Gary's Blastoise.
You know who actually wrecked among the Johto mons?
Wins: Ash's Bulbasaur(A solid win, it was stronger than Bulbasaur was after two seasons in it's very first episode) Ash's Charizard(technically a victory, though it got injured in the process) Wild Primeape(Defeated it in one hit, it was a leader of a fighting mon posse containing a Hitmonlee, Hitmonchan and Machoke) Bugsy's Spinarak Misty's Staryu Jessie's Arbok Technically didn't deal the last blow, but did defeat Jessie's Arbok and James Victreebel simultaneously. Chuck's Poliwrath Chuck's Machoke Harrison's Houndoom
Losses: Falkner's Hoothoot Bugsy's Metapod Gary's Blastoise(Bayleef's one-hit loss to Gary's Blastoise was bollocks, but eh) Harrison's Blaziken(Kinda the same here, but it was a bit more reasonable)
For people who hold Bulbasaur to be one of Ash's strongest Grass-types(it is strong, I agree)... Do remember that Bayleef was by all implications stronger than Bulbasaur when they got oaked at the end of Johto, and that despite claims of Bulbasaur being impressive in the Frontier, it only actually won one match(against Brandon's Dusclops, already on it's last legs after defeating Charizard), and tied another(against Brandon's Solrock. admittedly it was the one with damage taken, so I see this one as a + rathe than +- match for Bulbs).
Bayleef was the Johto Powerhouse, not Heracross, and I'd argue it should be the mon argued for top grass mon together with Sceptile-
And frankly, even this is a bit unfair to Bayleef, because Sceptile wouldn't even be debated to be as strong(or stronger) as Bayleef) if it weren't for the Darkrai win(which it had quite a bit of help for). Using that kind of logic, Torkoal, Ash's weakest Hoenn mon should be considered a lot more powerful, as it nearly defeated Brandon's Registeel. Yes, it has fought other legends as well, but never beaten them.
As for Swellow... Win Winona's Swellow Liza's Lunatone (Lol at thunder armor) Juan's Whiscash Katie's Venomoth Katie's Scizor Tyson's Hariyama(which stomped all over Cophish and Torkoal) Tyson's Donphan(oneshotted it after taking about ten Rollouts) Note: Do remember that Tyson is a league Champion. Tucker's Swampert Spenser's Venusaur(which completely outclassed Ash's Heracross)
Draw: Morrison's Gligar (Double KO)
Loss Anthony's Pelipper(but through cheating) Brawly's Machop Drew's Roselia -Swellow Time! Juan's Milotic Katie's Walrein Tyson's Metagross Spenser's Claydol
Pretty amazing. Note that almost all of its wins are against very high end opponents.