First time I ever disliked the anime is when Misty left, but I grew to like AG, and so I got used to it (didn't like character replacement back then).
I started losing interest altogether when the dub switch occurred and the show moved to CN. I still followed it, and really disliked it during that time period. Watching it after Season 8 gave me a terrible headache.
Middle of DP is when I stopped watching entirely. I didn't start watching until the end of DP again. So, I left during mid-Sinnoh, and returned during the end of Sinnoh.
I watch BW sporadically. I think it's alright, and occasionally good. Regained interest with it.