Snorlax contributed in Ash conquering the Battle Frontier most notably against Greta when Snorlax had to carry to clutch up the win.
Ursaring has battled against Brandon's Regice and although Ursaring loses he pretty much put up more of A fight than even Paul's seasoned Pokemon from past regions, without using Bulk Up or activating Guts.
Buizel was so good that he was commanded by Elite 4 Lucian and even beat Zoey when he was wild(all of this happened admittedly before he became Ash's 😅).
Oshawott did beat his evolved form in A scalchop duel, clutched up against Clay and beat up some Onix on Onix Island along with Piplup when Meloetta was having trouble(if remembering correctly 😅)
Corphish contributed in conquering the Battle Frontier even beating fully evolved Elite 4 Level beasts like Swampert and Arcanine with Swellow against Tucker.
Ash-Greninja battled on par with and notably damaged Alain's Mega Charizard even making him eat the dirt, the pokemon that's battled 50cent Zygarde and Primal Groudon.
Blazed-Infernape beat Paul's Electivire who's Paul's strongest pokemon even over Torterra, the pokemon that notably damaged and made Cynthia's Garchomp kneel. Cynthia's Garchomp battled the Forces Of Nature in their Thearian Forms in Black and White.
Not gonna lie, I think Greninja and Infernape going all out got it.
For me i'm giving Ash:
1# Infernape vs Registeel(no reason other than just to have my goat Infernape on the arsenal 😎, plus all of his moves are super effective against Registeel)
2# Sceptile vs Regirock(rematch, and no King of Pokelantus this time 😏)
3# Charizard vs Regice(call back to the Articuno W 😈)
I personally am going to go with Saracen, they have desert spinjitzu and imo are really cool in design. Plus last time I saw the Sandbenders they we're scum from what I recall.
Charizard has the feat of bodying an Articuno before specifically Nolan's friend Articuno back in the Battle Frontier.
Charizard has the feat of no diffing Iris's Dragonite in Best Wishes, Iris's Dragonite fought against the the Forces Of Nature in their Thearian Forms pokemon powerful enough to cause random natural disasters, but most notably Cynthia's Garchomp also was there and like Dragonite struggled just as much against the Forces Of Nature, yet Charizard folded Dragonite just as easily as the Forces Of Nature did.
Does Charizard also slam JN Articuno like he did with BF Articuno?
Infernape with Blaze has the feat of no diffing Paul's Electivire, A Pokemon that as an Electabuzz his second stage was the only Pokemon in Paul's arsenal to deal decent damage to Brandon's Regirock unlike Paul's older mons that's been to past region leagues.(This feat should be Mid E4 Level, because it's Brandon and Electivire did it as A second stage)
Infernape with Blaze has the feat of bodying Volkner's ace Luxray. Volkner is Flint's long time rival and depicted to be near Flint's strength level. The same dude who's Infernape was able to go to to toe with Cynthia's Garchomp after beating two of her pokemon before hand.(This feat should be High E4 Level to Low Champion, because Flint and Volkner are depicted to be on or near the same level of strength as each other)
So does Ash's Infernape with Blazed get's his get back against Moltres? WITH NO INTERRUPTIONS THIS TIME(looking at you Gary Oak)!!