>>57694409We can eliminate every protag who did not beat Red. This is because the protag is canonically stronger than the NPCs and is destined to beat them in a battle. So no champion or legendary Pokemon is stronger than Red, because Red is a protag, and player characters always beat non player characters.
This leaves us with Gold Nate and Elio or whatever the default name for sun boy is.
Golds other feats are destroying a weakened Team Rocket, defeating one of the weaker champions (he is weaker than Blue, but I suppose Gold also beats Blue too) and catching Ho-oh/Lugia both of which are eclipsed in power by stronger legendaries.
Nate destroys a weakened Team Plasma, defeats the fusion of Kyurem and Zekrom/Reshiram which should be one of the strongest Pokemon in the lore, as well as captures the unfused versions of them, and defeats Iris who is probably around Wallace and Lance in terms of lore wise strength.
Elio defeats Team Rainbow Rocket a multiversal threat, defeats Ultra Necrozma, which is probably the strongest Pokemon mentioned in this post and catches a weaker version of it, but everyone he beats in the Pokemon league is weaker than most champions.
I hate to let Gold down, but Elio is the strongest trainer canonically, MAYBE Nate but probably Elio.
Not Gold
Not anyone else