I'd say Alola had the highest average and overall strength. Given that it had a legendary, an ultra beast, an incineroar that could tangle with one of the strongest mons in Alola and a pretty solid Lycanroc. Even Rowlet wasn't that bad of a weak link by the end, given its use of feather dance and durability feats. I wouldn't be surprised to see any of them get a KO from anyone below the Elite 4.
Kalos would be second, with Greninja being incredibly strong, and none of them being laughably weak.
The presence of so many flying types would be a problem if A. they didn't have secondary typing to reduce their weaknesses and B. The anime actually cared about such things. But neither were the case.
The team against Gary is third. Snorlax was already ludicrously tanky and Charizard had had its valley training. Ash's strategies were quite effective. Heracross and Bayleef had become fairly heavyweight while Muk has a special utility against speedy pokemon.
His team vs. Paul would be a very close fourth. Infernape was strong, but wound up pulling much more than its own weight. Which makes the team somewhat fragile if Infernape can be statused. Torterra in particular was a bit of a lodestone. Buizel was quite ineffective as well, in both the early full battle and the final league battle.
Totodile and Torkoal kinda bring the average down for Johto and hoenn teams respectively. Several of the Orange League mons have gotten stronger since that battle. Against Tobias; Swellow, Sceptile and Pikachu were great, Heracross was decent but Gible and Torkoal were always unreliable. The Unova Team was his biggest joke since Kanto.