Don't we already have Dead Homers Society and numerous other threads around here for this?
Either way, I honestly consider all seasons past 10 or so to be below-average to awful (23 and 24). The show has, by means of its overly long-running state, essentially degraded in these stages:
Season 9: First season to feature weak episodes. Story coherency begins to slip. Origin of Jerkass Homer.
Season 10: The beginning of the full-force decline. A major drop from season 9 and dominated by Scully's vacuous wackiness intemingled with fleeting moments of classic Simpsons. Jerkass Homer becomes the centre of the show's attention.
Season 11: The death of the show's quality. The wackiness is further increased and the remainder of the cast begin their flanderization.
Season 12: The Scully era reaches its nadir, with incoherent plots and an absurdly obnoxious tone.
Season 13: Al Jean returns, but worsens the show in some ways, replacing the zaniness of the Scully era with crushing blandness and safeness which render the episodes less watchable.
Seasons 14-16: The show attempts to return to the classic formula, with several plots being conceptually more promising than the Scully era, yet the execution and humour of the show gradually dies even further, leaving a painfully unfunny and sterile mess.
Seasons 17 - 19: The extinguishing of any remaining hope for the show. The episodes degenerate into white noise/dullness.
Seasons 20-24: The deepest, darkest recess of the show's run. Virtually all the humanity has now vanished in favour of dull humour, painfully written dialogue and awkward execution.
Seasons 25 - 30: Has yet to fully reach the worst of seasons 20-24 (although season 28 is a close contender), but remains similarly bland. The show has essentially ceased being 'entertaiment' - to me, the show has basically devolved into a bunch of ideas and scenes pasted into a 20-minute compilation to feed FOX's demands. Some may contest that the early Jean (13-16) seasons mark an improvement over Scully's, which I can understand, although there seems to be some intangible quality about even the season 13 Jean episodes which create a soulless, safe air to the extent where they become genuinely difficult to watch without becoming drowsy. Wasn't this once a show which balanced innocuous plots like Lisa getting a rival with amazing memorable gags like Milhouse being pursued by the FBI and Ralph's bizarre victory?