I stopped mid-Johto I think. The show advanced at the pace of a snail back then.
But really, I stopped watching because of Ash's plot armor. I know Pikachu is supposed to be the mascot and all that, but really, it's way too strong in the anime. The anime depicted things that puzzled me back then. Ash's toddler Pokémon could beat things three times more powerful than them. Also, the whole thing with "being friends" with Pokémon and what not, being stubborn about things such as evolution, and so on.
Finally, every episode is the same apart from a few different elements. I started watching the first series of Yu-Gi-Oh at the same time Johto was airing in my country and the storyline approach of Yu-Gi-Oh appealed to me much more, despite Yami Yugi having way bigger plot armor than Ash.
I was really happy when I found out they introduced in DP a character with the mindset of a competitive battler (Paul). Paul was the best invention they could've come up with, a guy who does exactly what we do at the beginning of the games (something like, catch three Starlys and keep only the one with a good nature). He was a metaphor, an antithesis to the whole anime and the message it passed to the viewers, and, since I was fed up with Ash, I thought it was a great thing. Paul is what the kids grow up into once they grow out of the anime and take the games more seriously.