The show had a noticable downturn in quality around season 12.5 for me. But I can pinpoint the exact episode where the entire thing jumped: S13E06 "She of Little Faith". Lisa became completely insufferable at that point and the show took a big hit for it. Also the first season I didn't keep the Treehouse of Horror from.
In my opinion, the five worst seasons, in order: 17, 20, 1, 19, and 13. I do give the first season a lot more respect than the other four in that list, but it wasn't really the same show it became even just a season later; it was written more like an 80's style family sitcom. I've kept only four episodes from it, all of them for classic moments / scenes.
I find season 18 to be quite the anomaly though. While no return to the golden era, its a noticeable improvement from the seasons surrounding it. Easily one of the better post-gold seasons.