Kanto: Top 4 at least.
We don't know the complete level of competition in the final rounds here due to only seeing Ritchie vs. Assunta. But considering the sheer amount of unevolved mons
as well as Ashs increased experience since the Kanto League. I don't think it'd be a long shot to say he could win. But since we don't know, I'm sticking to the top 4.
Johto: Win.
Factor here is how much more useful would Blaziken be than Houndoom in Harrison's battle against Jon Dickson. Using Ash's Bayleef as a measuring stick helps show
the gap in those two mons power as Bayleef was able to easily defeat it, while Blaziken took down Bayleef effortlessly. So if Blaziken was used, it would have probably
kicked the crud out of Dicksons Rhyhorn and won him the tournament in the finals. Given that the level of competition was low enough for a trainer slightly stronger than Harrison without Blaziken to win the whole thing. Kalos Ash would probably mop the floor with the competition. Heck, Johto Ash with a healthy Charizard probably could.
Hoenn: Top 4 at least.
Considering how close Ash got with his Hoenn Team. I wouldn't be surprised to see Ash beat Tyson in the top 8 with Ash Greninja to take out that Meowth. Pikachu is the Anime's
ultimate Metagross counter, so that would work in his favor too. The only other threat on Tysons Team was really that Hariyama that took Swellow to beat it. But considering that
Ash has three Flying types to take it out, I wouldn't see him having trouble with it too much. Sceptile, Shiftry and Donphan, on top of totaling half a win altogether, are weak to
Ash's Flyers or in Donphans case Greninja. So I don't think Ash would have a hard time against Tyson with his Kalos Team. The opponents after that are the question.
Sinnoh: Top 4.
Its easy to just say that he'd lose just because he's facing Tobias. But also considering that Ash wouldn't have his Reserves backing him I can't see him getting past Tobias's
fourth mon. Best case, the Flyers and Goodra combine to beat Darkrai. Pikachu ties with Latios. And Greninja manages to take out a third mon of equal power to the first two. (Cause lets be real. He had to be pretty powerful already to catch those Legendaries.) I just don't see Ash-Greninja being able to power through four mons to win the match.
The form would take too much of a toll on Ash's body most likely; losing to Tobias in the process. Or maybe Tobias really did have four Magikarps splashing around in his party after Latios went down.
Unova: Top 4 at least.
Ash could beat the pants off of Cameron with Greninja and Pikachu alone. Next: Virgil. Probably a Harrison/Tyson level Trainer. If so, I think its likely Ash beats this guy. However, since they never battled its impossible to determine the outcome there. However, if we take into consideration the fact that Ash was able to defeat Dino (Albiet in a sample size battle) and that Dino was able to take Virgil to his last Pokemon. Its definitely possible for Ash to win the whole thing.
Aside from Ash's lineup though. The other change is obviously Ash's ability to strategize that has gone under change. Heck, in the Alain battle alone, Ash was not only able to
think on his feet at a seconds notice, but also trust his Pokemon to get the job done. These traits alone make me wanna say that he could win all of the above leagues except Sinnoh.
Probably a bunch of holes in my logic though. But hey, I'm sleepy and its 1AM here. What are ya gonna do?