The strongest trainer is whoever is being played. If Red were still the strongest Pokemon trainer in the world, he wouldn't have lost to Gold even after 3 years of solid training on Mt. Silver. He wouldn't have been beaten by Nate in the Pokemon World Tournament.
Red and Cynthia are the two strongest (non-player) trainers in the mainline games, by the numbers. Cynthia's team is slightly higher average level, and has perfect IVs/EVs. But, if you want to argue lore, I'd say it's either N or Volo - the two trainers that have legendary pokemon on their team.
wWhMxex. 218 307 173 319 496 428 191 499 217
pokemon is red the strongest trainer
