I think 12 was where I started to struggle a lot.I've been ploughing through them on Disney+ starting from the very beginning and I'm currently at the end of season 13.
Season 9 is when the cracks started to appear (still a great season for the most part). Season 10 is when it started to feel like a different show. The characters changed, especially Homer infamously transitioning from a dim-witted yet kind-hearted family man to an a-hole (Jerkass Homer) and the plots becoming more nonsensical. Basically became less of an animated sitcom and more of a cartoon.
It's still entertaining, although inconsistent, but I'll persist and see how far I can get before I feel enough is enough.