Good question, actually...
1. Kalos Saga
2. Advanced Generation
3. Sun/Moon
4. Diamond/Pearl
5. Best Wishes
6. Indigo League
7. Orange Islands
8. Johto saga
Honestly, Kalos was the best saga. I think the character dynamics in that particular season were very strong, and kept me interested in that entire arc in particular. The animation quality was also top notch here. Some of the battles were very well done, I don't remember being as excited to watch a fight since the Sinnoh League.
Advanced Generation is second because I liked how Ash became a mentor role to a new trainer, and the story wasn't just focused on him and his travels anymore. Also, we got introduced to Contests, and I really liked that it was no longer just Gym Battles. Also, the Battle Frontier arc was a fun journey, even though all the voice actors changed in that season...
I like Sun/Moon maybe just a little bit more than D/P because the animation is so much more expressive, and I just genuinely enjoyed S/M. I know it has its share of haters, but when I saw the new animation style, I really enjoyed it. Also, Ash won a league that's canon to the Pokemon games, which was a first. Keep in mind, though, since I keep up with the dub on a Saturday basis, I haven't seen Ash win yet, but yolo.
Diamond/Pearl saga was great, and it was actually my first Pokemon anime way back in 2007. I started watching Pokemon weekly after the April "movie" version of this came out. While I do love Diamond/Pearl, I felt the dub's script was pretty awful, and that some of the gags in this series were especially dated. Ie: Brock still falling in love with every girl he sees. Also, quite a lot of the filler in a row was unnecessary and dragged Sinnoh out longer than it needed to be. Regardless, I do still enjoy this series. I liked Dawn, Ash & Brock's respective character developments.
I also liked Best Wishes, but just.... not as much as D/P? I do mildly agree that Ash being dumbed down in this saga was a little annoying, but I don't know, I got over it quickly. There were some pretty stupid decisions made by Ash, though, I honestly thought he caught way too many Pokemon in Unova. The amount of Pokemon he caught made it really hard for me to care about all of them. Also, the fact he lost to Cameron in the Pokemon League still kinda pisses me off to this day. However, for what it's worth, I did enjoy B/W. Sure, it had a lot of bs, but I guess I'm just bias bc I like the Pokemon anime a lot.
I did like the Indigo League, too, but Ash was a bit rash & made a lot of rookie mistakes here, but... what can you expect, really? Ash was a rookie here. While Indigo League was a strong season, there were some really weird filler episodes, some that are extremely infamous by now, such as the Beauty and the Beach episode. But this season established a lot of important things, and this is where Ash's journey started, so I could never consider it my least favorite.
...Although, I guess same can't be said about Orange Islands? Weirdly enough, this used to be my favorite season of the anime. Upon rewatching this season last year, though, it was honestly just way too short. I don't dislike Orange Islands or anything, I still like it, but I don't know, it just feels awkward and just there as a way to fill in time before the Jotho games were coming out at the time. I guess that's why I rank it lower than the rest of these.
And finally, Johto. Johto while, I did like that season, is definitely my least favorite. My major problem with the Johto saga is the major blocks of filler in between gyms. Some of the gaps between gym battles were way too huge, and for absolutely no reason at all. Come on, was the Whirl Island Arc even necessary? Not really, no. The filler of this season was just too much, even Sinnoh's huge filler gaps weren't as bad as the filler here. Filler aside... I did enjoy Johto for what it was worth. Ash had major character development, and I did like his Pokemon that he had in this generation. I just wish the Johto arc wasn't so dragged out. If it weren't I definitely would have liked it more. Also, Ash's Bayleef is precious.
I can't rank Pokemon 2019 yet since it's just recently started, and I need to catch up to it. I'll probably do that tomorrow. Although it's too soon to rank it, I'd say.