The Simpson's couch gag might get most of the attention, but the show has had a number of hilarious gags over the course of it's two decade run, from Bart's chalkboard gag to altering the names of the cast and staff in the "Treehouse of Horror" episodes. One of the more recent (and short-lived) gags is the billboard gag that began appearing in season 20. The show had its share of clever billboards before bringing the joke into the show's opening credits in 2009, but between seasons 20-23 fans were treated to an extra joke crammed into the show's iconic opening sequence. 

Behold, the 16 greatest billboards that Springfield has ever seen, counting down with... 

16.

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Source: simpsonswikia