Jouto and Sinnoh are at the top for me, since those are the two leagues where Ash truly went full power by using both his regional teams and his reserves, and Ash had decisive victories over established rivals like Gary and Paul, the former edges out the latter mostly due to Tobias, a guy with no backstory suddenly appears with legendary pokemon just to keep Ash from winning? Really writers?
Hoenn would be next due to having Ash test the waters with only a regional team for the first time since Kanto, and did have some full battles, granted Ash's fight with Morrison was pretty stupid, due to his reluctance to fight Ash at first costing him half his team, and then Ash got dumb too by sending Pikachu against a Steelix, but other than that it was still pretty good.
next is a tie between Alola and Kanto, Alola being the first league Ash ever won, but the lack of qualifications and full six battles during the league did undercut the intensity factor in my opinion, where as Kanto was Ash's first league under similar pretexts as it was his first one(it had full 6 v 6 battles, but Ash did not get that far), not to mention both leagues had some controversial call moments like Rowlet vs Decidueye, and Ash vs Ritchie in kanto, though more so for Squirtle being unable to battle by being put to sleep than Charizard refusing to fight, I can understand why a judge would make a call like that because if it's not going to even try and battle then why let the match continue? Regardless this is where they will stay ranked.
Kalos is next, i'm sorry but they rushed the heck out of this one, considering how poor Goodra did, it really did not need to return, and Ash-Greninja's pay off not helping Ash in the end really sealed the deal(granted at the same time i would've considered it cheap if that Fanfic style transformation did win Ash the league since I've never liked it to begin with), still it's better than the last one
Unova is dead last, we may have had more onscreen matches than the above one, but the only real good one was Ash vs Stephen, taking Trip out in the prelims was a joke (even if Pikachu deserved the karma factor given what went down when Ash arrived in unnova), especially with how it was handled, and then there is the Cameron fight, thing is though i'm not at his throat for bringing 5 pokemon to full 6 battle (especially since once we found out what his sixth was, a Watchog, Ash probably would've beaten it) but rather the stupidy between both of them in the battle was even higher than with Trip's Serperior losing to Pikachu, since at times they would send or leave out pokemon who clearly had no chance against the enemy(like Oshawott vs Hydreigon and ferrothorn vs Pignite), thus leading to some really stupid situations in that battle, and of course building up Virgil as a rival, and not having Ash getting to face him was also a poor decision.
so there's my two cents on the matter.