By "greatest" do you mean "funniest"? I'd like to see a little blurb after the clip of why this made the cut (revolutionary / often quoted / spurred copycats, etc).
What makes a joke GREAT? Hmm. I kinda feel like I know it when I hear it. Tough to define. But let's try...
First of all, it's gotta be funny. Really funny. Then other stuff comes into play: how much of a point there is to it, how much it reveals something about the comedian, how well it's executed, how it's performed, and how unique it is.
There are things that I laugh at a lot but that I don't think are really "great" jokes. Sometimes they're just silly. Nothing wrong with that but, to me, the best standup is hilarious AND has a point too.
To go back to that list, these were jokes that all made great points: Chris Rock's Black People vs. Niggaz, Louis CK's Why?, George Carlin's God, and Doug Stanhope's Fuck the Jews.
But not every bit on the list was making an important point. Sometimes it's more the craft that turns me on. In Bernie Mac's You Don't Understand, the attitude, rhythm, and performance of it is incredible. Paul F. Tompkins' Peanut Brittle is great for how deep he goes into such a silly topic and how he acts the hell out of it. Jim Gaffigan's Hot Pockets is like a clinic in how to get all the comedy meat off the bone. He just hammers every possible angle. Andy Daly's Knock It Off perfectly deconstructs the silliness of typical hackery by going for minutes on end without ever actually saying anything.
I guess for those bits it's the artfulness of 'em. Same thing with a guy like Hedberg. He's not saying anything important but it's the way he sees the world and how he expresses it that makes his stuff so amazing.