Sinnoh > Alola > Johto > Hoenn = Kalos > Unova > Indigo
Sinnoh was the most consistently good and still has my favorite battle/episode in the anime by far. Basically every other battle ranged from good (Nando, Barry/Paul) to great (Conway). And as much as I get the hate for Tobias, I can't help but kind of respect the sort of "blaze of glory" aspect to it. I can appreciate it for what it is since it's obvious the writers knew Ash deserved to win.
Alola has its problems, but its peaks are excellent. I also think some of its controversies are really overblown. For instance I don't see the problem in the lack of requirements when it's still clear that the highest-ranking competitors were incredibly powerful. Ilima canonically entered the Kalos League, Guzma wiped the floor with him, and both Ash and Gladion were even tougher than that. I definitely have issues with Hau's battle, and Gladion not being a 6v6 is kinda lame, but the good far outweighs the bad for me. I also do count Kukui, which is easily a top 3 Pokemon battle.
Johto's solid for what it is, though it's definitely a good deal below the top 2. Funnily enough I actually prefer a lot of the other battles to Ash vs. Gary. I just really don't care for how formulaic that battle was with the "Ash makes a comeback by OHKOing the opponent!" spam. I still like the outside interactions between Ash and Gary though, and it was a solid League overall.
Hoenn and Kalos are about equal for very different reasons. Hoenn's just kind of eh. It's got some enjoyable parts but AG's lack of a rival means there's not much tension, and since it's AG the animation was pretty lackluster too. Kalos hits much higher peaks thanks to its stellar animation but also way lower valleys (insanely rushed, Goodra/Noivern getting no wins, using the hype train as a crutch). Can't decide which is better or worse.
Unova is bad but just slightly edges out Indigo because of Ash vs. Stephan, which is legitimately a fantastic battle. Everything else sucked.
Indigo's my least favorite. Horrible animation across the board (yes I know it was the anime's first season, but that doesn't change the quality lol), and I think the concept of Ash relying on beginner's luck was a good idea that they screwed up at almost every turn. What does catching an overpowered Krabby have to do with beginner's luck? How it is karma that Team Rocket forced Ash into an unwinnable situation with Ritchie? The only battle that even kind of competently executes the "beginner's luck" theme is Jeanette's, and that still has its problems. Not good at all.