Prior to Journeys, the obvious choices would be Pikachu + the regional ace of each series:
Starter: Pikachu
OS: Charizard
AG: Sceptile
DP: Infernape
BW: Krookodile
XY: Greninja
SM: Lycanroc/Incineroar
Also, each region sort of had a secondary ace as well:
Kanto: Bulbasaur
OI: Snorlax
Johto: Heracross
Hoenn: Swellow
Sinnoh: Gliscor
Unova: Pignite
Kalos: Hawlucha
Alola: Lycanroc/Incineroar
Of course, the JN team trumps everyone based on feats. Prior to Journeys, you have Charizard and Sceptile taking down powerful legendaries. Ash-Greninja applied heavy pressure on Diantha's Mega Gardevoir and Alain's Mega Charizard X, but Greninja never got any actual wins against Elite 4/Champion Pokemon. The rest of Ash's Pokemon took down Frontier Brain Pokemon and/or league conference champion Pokemon, but never actually beat an Elite 4/Champion Pokemon.
The JN team consistently beating these Pokemon basically overshadows any other feat. The only in-universe explanation, IMO, would be that JN Ash has gotten even better from DP/XY/SM Ash to the point that he is now Champion-tier, and therefore any Pokemon he now trains in a region will improve exponentially farther than whatever he did in prior regions, even if that is never shown on-screen. Personally, I don't think it makes a whole lot of sense. I would expect some of the reserves, especially the more battle-tested and powerful ones, to be this strong at this point, but not the entire JN team except for Lucario (who's meant to be ace of this series).
It's hard to really compare. Here's a thought experiment: If JN Ash didn't capture any other Pokemon, but he only used his reserves this series, would they all have been upscaled? At the end of the day, the writing team wanted JN Ash to become the #1 trainer in the world, so they made his team that level.