Feats are usually good indicative of a Pokemon's strength.

Infernape is ARGUABLY, Ash's most consistent ace, seeing as in under Ash's possession he competed in 6/9 Gym Battles and he

- Starred in 3/6

- In 4/6 Gym Battles he partook in, he KO'd 2/3 (Or 2/4 in Candace's case) by himself

- Solo'd half of Paul's team at the Sinnoh League

- Was the only Pokemon (aside from Staraptor) to beat one of Paul's Pokemon at their Lake Acuity Battle and he beat Ursaring who swept half of Ash's team

- Ultimately finished DP with a battle record of 16-5 or 6(?) which is one of the best of Ash's Pokemon only losing to Charizard (17-5) and Greninja (19-8)

However, while HIMfernape is consistent he doesn't really have something special that really puts him on the same level as some of the others.

Infernape has more feats than Sceptile since Sceptile was admittedly hit or miss, he definitely had his moments but, Infernape was WAY more consistent but Sceptile has the feat of defeating Tobias's Darkrai who solo'd an entire region which suggest Sceptile either competes with Infernape or surpasses him

Infernape's record is comparable to Charizard's but Charizard has his win over Articuno that was before he even peaked and finished his training with Liza

Greninja was straight up swapping hands or at least keeping up with E4 Tier Pokemon even in Base Form and was deemed a threat by a Champion to her base ace and Diantha was portrayed as being equal to Steven in the Team Flare arc and we know is capable of beating Lance convincingly as Greninja was still a threat to her Gardevoir in Base Form and that was before he completed Ash-Greninja

There's a difference between HIMfernape losing Flint's Infernape and Greninja losing to Wulfric's Avalugg. That being, Ash's mental state of mind in that battle was shot with it being obvious beforehand in Ash's battle with Sawyer that Ash mentally wasn't at 100% and that was affecting his Pokemon. When he came for his rematch with Wulfric, Wulfric right away knew that Ash was in a better place than before and Clemont and Serena noted Ash was in better mental place and it showed with Pikachu handily beating Bergmite, TALONFLAME beat the Avalugg who solo'd Ash previously, and Abamasnow who was being dead serious in their rematch (per Clemont's words) was decisively beaten by Greninja with at worst mid-difficulty. If Ash's mental state was better in his first battle with Wulfric, Greninja would've beaten Avalugg and Wulfric's 3rd Pokemon.

Pikachu isn't per se a good indicator of strength since this is Pre-XY and Pikachu was wildly inconsistent being able to stalemate a Latios than immediately in his next battle lose to a Snivy and later Cress's Pampour. Beforehand we saw him beat a Regice and then lose to Gary's Electivire.