So, by this I mean, out of all the regions, in which one do you think he had the strongest team. Please take into account all the Pokemon he caught in that region. Think of all the Pokemon's strengths and weaknesses. Also, if he didn't catch the Pokemon in that region, but he used it in that team, (e.g. Donphan in the Hoenn Team) then it counts as a member of that team as well as the region it was caught in. However, leagues and the Pokemon he used the Battle Frontier matches (The ones that were with him all the time count for BF season. e.g. Sceptile. BTW Battle Frontier Team counts as Hoenn Team) don't apply to that rule (e.g. Quilava was used in the Sinnoh League, but it wasn't part of his Sinnoh team). Although you can take into account the moves it learnt during/prior to the leagues (Quilava with Aerial Ace). And please give a reason for you choice. (Sorry if this confused anyone, if you have any questions just ask me).
I'd say Hoenn. Kanto may have had the most powerhouses, but most of the Pokemon didn't get enough screentime to truly determine their strength. Sceptile and Torkoal were also 2 of the 4 Pokemon that beat Tobias' Darkrai. And Swellow must've done quite a bit to Latios coz Pikachu knocked it out (although it got knocked out too) and Pikachu was used straight after Swellow. And Glalie did pretty darn well in the Hoenn League too. The overall strength seems to be the highest too IMO.