Geez, it's been so long since I watched the anime that I can't honestly remember a whole lot about it, where I stopped exactly, or even the definate why. Here's what I can remember.
I started when it first aired, back before Kids WB got a hold of it, back when the episode titles were just as descriptive as the original Japanese titles. And I remember loving those episodes the most, because 4Kids was a really, really small company back then, and they seemed to be rather humble. The episodes were treated with a certain kind of respect I feel dropped once 4Kids got bigger.
But honestly, I have no beef with 4Kids. I'm far too old to, for one, and through and through, the Pokemon anime was still one of there more faithful works compared to game-animes like Kirby and Sonic X.
Anyway, my sister and I were so on top of the Pokemon anime, it's what convinced us to buy Red and Blue. My first episode was the one where Ash dresses like a girl to fight Erika... I think. The grass Gym Leader at any rate. I thought that was good old fashioned comic-anime gold. My favorite episodes are probably Pokemon Fashion Flash, and the one with the giant fake Pokemon and the Pokemon themselves talking in subtitles. The one that followed, Tentacool and Tentacruel, was pretty neat, too. Aww... now I want to watch these again. I think I was so upset that there were no more episodes after a point. Talk about a cliffhanger, I never thought I'd see the rest of the series. :(
Then it got picked up by Kids WB, and I was estatic. Problem is, now the Kids WB years are one big mess of a blur. I recall only two episodes. Pikachu ReVolts for being an episode done by the "good" animation studio, and Wherefore Art Thou Pokemon because I love me some Nidoran (by extension, I only recall the Nido parts of the pink Pokemon episode, the "Pinky Land" dream sequence, something about Rhyhorn and nothing else). The Johto episodes are when I started to feel some fatigue. Through and through, I liked the Indigo and Orange Island episodes even if my memories are hazy, but Johto was brutal, and I swear the animation quality started to drop. I only remember the Larvitar episode (vaguely), and Squirtle making a comeback in the Johto League. And Chikorita/Bayleef having a crush on Ash. Oh, and Whitney having a Nidorina who was on screen for 5 seconds. :P
Here's the thing, most everyone I knew gave up halfway or even partway into Johto, and I can't blame them. But I presevered, and why? Because the Team Rocket centric episodes (that I don't even remember -_-, though now I'm starting to recall Holy Matrimony) were some of the best character development stories in the series. That, and watching the show just for TR's sh-tick alone gave me a reason to continue. Granted, I started to develop a disliking for Ash's Blast-'em-Now-Ask-Questions-Later mentality towards TR, but I could totally understand that his tolerance level for them was at zero some time before they got to Houen.
Advanced Generation sort of picked things up in the first few episodes. The new digital animation felt inspired, and the quality went up, if only for a short while (they actually started to blink in scenes when they stood around listening to exposition. Amazing!). I loved May and Max, and thought having that brother-sister dynamic in the show was cool. On top of that, I really enjoyed the 3rd generation of Pokemon, and the games way more than generation 2.
And yet, somewhere during Advanced Generation, I just dropped off. It had TR, May and Max, a set of Pokemon I actually liked, but it still wasn't enough. I think it just became too formulaic for me. That, or I was flat out fatigued. Either way, I don't know where I stopped for sure, it was after the Ralts episode, and they weren't done getting all the badges/ribbons/medals/whatever.
I remember catching an episode of generation four where I think Ash's Turtwig evolved into a whatsits (I stopped playing at Gen 3), and all I could think of was "it's still the same old show". Like The Simpsons. Except The Simpsons is probably much worse.
So, that's my long and rambling story, which triggered some fun memories along the way, so I apologize for getting wordy. But yeah, overall, I stopped during AG due to fatigue, or not buying into the formula anymore.