By regular trainer I mean like an ace trainer, or a fisherman, not someone special like the champion, red, frontier brains, etc. This question includes all games, and challenge mode doesn't count. Thanks! :)
Well I'm not sure if the White Tree Hollow and Black City are excluded from the banned list because the highest trainer would be Boss Train in a tie between Benga, Janna, and Jariel. All which have a respected team of Lv. 80 Pokemon in Black 2/White 2.
If they aren't included then the highest trainer would have to be a tie between 2 Veteran in Black 2/White 2 Route 18. Veteran Vincent and Veteran Maya who each have one Pokemon at level 70.
Note: None of these are Challenge Mode levels.