Sorry to be buzzkill, as I'm debunking something I've seen various content creators obsess over and do a lot, but the idea of determining who the strongest trainer is in Pokémon canon is ridiculous as Pokémon's canon is extremely loose. Much more loose than what at least 95% or more of Pokémon fans want to believe. It spans throughout an entire multiverse of infinite universes and infinite different possibilities, including each of our own personal experiences in our own save files as their own individual universes each. The latter was confirmed by an interview with Satoshi Tajiri in 1997 and an Entralink feature in Pokémon Black and White. The former was confirmed by the combination of:
1) Celestic Town lore: Stating that Palkia not only embodies all the space of a universe, but the material space of all dimensions
2) Ultra Space: Being a dimension outside the planes of a universe, connecting countless amounts of wormholes that mostly all lead to different universes. This is essentially the multiverse and because it links all the Pokémon universes together from a dimension outside them, Palkia embodies the Pokémon multiverse, with Ultra Space merely being an extension of Palkia itself.
3) An interview with Junichi Masuda, explaining that space-time in Pokémon is infinite. This by extension means Ultra Space is infinite, confirming the Pokémon multiverse is infinite in size.
The latter is also backed up by:
1) The TCG backing up the concept of infinite time and infinite space within Pokémon
2) Palkia and Dialga being able to create a universe in the anime and nearly doing so in the games, indicating how they are both beyond the limits of just a universe.
3) Pokémon Masters EX (a game that is confirmed to cautiously have only things mainline game characters would do and say) having Zinnia explicitly state that Pokémon has infinite universes.
When many people try to determine who the most powerful trainer is, they ignore trainers who are actually canon but may not seem like it. Everything in the canon is canon, but people tend to nitpick what's canon and not canon within the canon despite that. For example, All Secret Base and Super Secret Base trainers are canon (we can all be canon within someone's own game in Pokémon), all Battle Facility trainers and trainers we battle online are canon, every character from games whose existence is recognized by main series, TCG, or anime is canon, including
Mysterial for example. I've seen people try to frame each as these as "non-canon" to the series. That's not how Pokémon's canon works. Even every trade, transfer, and event distributed in the Pokémon series is canon in Pokémon. Every single one of our personal experiences are officially recognized within the series.
I am not saying everything Pokémon is canon. The official canon in Pokémon consists of the main series games, the anime, the Trading Card Game, and the manga. The games officially recognize the existence of Pokémon anime and TCG universes through media from said universes of showing up in the games in some way, usually in the form of an event, meaning they are all canon to each other. Any piece of media that can be recognized by the canon, is canon itself, as canon is aware of their existence. This includes the movies, a handful of spinoffs such as Pokémon Ranger and Pokémon Conquest, and side series games that allow you to directly transfer Pokémon to or from main series games, like GO, Colosseum, Battle Revolution, and XD. Anything that's not recognized by canon, is what's not confirmed to be canon. Correct me if this is false but Pokémon Mystery Dungeon has not been recognized by any sort of official canon media for Pokémon, and is thus not confirmed to be canon, but Nobunaga and his Shiny Rayquaza from Pokémon Conquest were featured in both the mainline games and the TCG, confirming the existence of Pokémon Conquest universes to canon, and making Pokémon Conquest canon.
What does this mean for trainers?
Your friend Jimmy down the street from you could be the world's most powerful trainer in canon. Maybe it could also be a consistent VGC player, like WolfeyVGC. Want to measure just NPCs? There is a guy in Pokémon Battle Revolution with Palkia, Kyogre, Mewtwo, and Lugia, but also players who are made as NPCs in other people's games in Secret Bases or Super Secret Bases. There's a random NPC in Sun and Moon's Malie City Pokémon Center who claimed they got wonder traded a Mewtwo, similar to how Super Secret Base trainers in ORAS occasionally tell you the latest Pokémon they received from Wonder Trades that actually happen.
Champions can't really be pointed out here either, as an NPC in the ORAS Battle Institute realizes then states that there are many trainers across the world who are more powerful than Champions, as being more powerful than Champions is the requirement for trying out the Institute. Anyone could be the best, especially if they have the best Pokémon, but it's likely never a notable trainer within the story of a Pokémon game, especially since they've historically had teams lacking for what is realistically possible in Pokémon games, and
random battle facility trainers show this off.