Originally I thought it was only good up until Season 9 (NY, the gun, The Leader, Joining the Navy, Garbage Commissioner) and thought most of Season 10 sucked at the time it originally aired (especially with the far superior Family Guy hitting the airwaves halfway through), but in light of later episodes, I'd say Season 10 was the last decent season, even though it began the "Homer gets new job every week" trend (e.g. inventor, celebrity butler, bodyguard, trucker, artist)
Season 11 started off allright with the Mel Gibson episode, but descended quickly with the Ritalin parody and Homer-as-food-critic, then really fell off the map with the first bad ToH and finally with Tomacco, which I always point to as the episode changing my view on the show's progession