Oddly Simpson's isn't even the show I go to when I think of that big iconic joke. Instead I think of the Futurama episode with the monkey Gunther and the profs line "Why? Why? Why didn't I break his legs?!"
In fact it's always Futurama that comes to me over Simpson's for quotes. I think the show had a way bigger impact on me than Simpsons ever did despite growing up with the latter for longer.
Pretty safe answer but I feel like Homer jumping Springfield Gorge is the definitive joke of the entire show. A joke so good it works any in language or culture it's translated into, and would probably still be funny in mute.
"Shut up, brain, or I'll stab you with a Q-tip!"
"Eat the pudding! Eat the pudding! Eat the puddig!"
"No TV and no beer make Homer go crazy!"
"You see, I was pressuring my wife to make love in front of our son, you're gonna laugh when you hear this, when suddenly..."
There are at least four or five moments in "Bart the Lover" (the Swear Jar episode) that are as good as, if not superior to, Steamed Hams. Not counting the already posted Swear Jar bits, it's either this:
Though the second clip excludes Marge's response, "For the last time, I'm not putting that in." That part absolutely KILLS me.
Also no, its not the funniest scene, but it has been the internet's favorite one to make videos out of for a little while now. Personally I think the funniest ones are the ones where the whole town is involved... So take your pick there...
I still call Hamburgers sometimes steamed ham because of it.
But for me one of the best Simpsons scene (because there are too many to call just one THE best) is the boogeyman one. I can't stress out enough how freaking hilarious and genius this scene is where Homer is alone with his kids at home and goes on panic mode after everyone tells a nightmare story. The delivering, the writing, the jokes. Everything is so good and is a prime example why the old Simpsons are so freaking legendary. Someone had even a picture of this scene as avatar on the old forum. It is that good.