I apologize for what i said earlier, and yes i do mean the adventure manga, something i should've elaborated more on (lol still pretty new so I'm still getting used it). And my argument to Platina being stronger than Ruby is that Ruby only started focusing on contests because he wanted to be a more sympathetic trainer, like his chilhood friend was (saph), but when he actually battled no one could stand in his way, his loss to Emerald was plot driven and was to make Emerald realize his bond with his pokemon, and towards the end of that story arc, he even taught Saph and made her a stronger trainer, showing that he can teach battling as well.
Though, like you said, We're only talking bout the dex holders, and a few non-dex holders that would be an interesting match for him i would say are Lance, and possibly Cynthia, but she was hardly given anything, especially with her team. Another possible choice would be his father, who taught him a lot of his battling techniques.