1. Johto-Notable for having 2 of the best 6v6s we have ever seen and most of all because Charizard was the main pokemon not Pikachu. Also the Gary battle was the only one where Ash went all out with his selection of pokemon and harrison's was just fast paced and frenetic. Plus it had a unique score system and aside from bulbasaur and squirtle stealing too much glory from his johto team in the prelims it was excellent.
2. Sinnoh- Ash had 4 battles, but all were excellently done from the Nando Bug off to Conway( AKA trainer which follows the games the best) using all sorts of strategies from trick room to power trick as well as Draco meteor it made things a real treat to watch. Ash and Paul was a 3 part battle(only 1 to date) and surprisingly there wasn't much filler or outside interruption. trollbias ruined it though.
3. Hoenn- While Ash fought the most and his 2 league rivals were interesting and 3 6v6s were always welcome, I sometimes had to pinch myself in disbelief at how sturdy his team was and how much damage it could dish out vs much tougher species. So if everything was the same and Crawdanut and sceptile was there instead it would have been higher.
4. Unova- 3 battles, the main rival gets kicked out early, random mid league filler in an already short league, Ash losing vs the dumbest trainer ever. At least Ash vs Stephan and the first half of Ash vs Camoron was decent
5.Kanto- Kingler was awesome, I don't remeber much else, but Ash's last battle was so bad to this day nothing could top it.