The Homer vs Bear episode might be the worst one they ever did. The flashback one with Lisa as president and Bart as a pothead slacker while Homer and Marge look for gold (Although the only part to make me laugh was when they acknowledged how dumb it was) is up there.
They should stay on the air. A couple of completely awful episodes aside (Ralph for President), the past five years have been borderline great. I've laughed hard during most of the episodes at least once. Nothing near the level of Grandpa's onion belt, but it's still funny.