I'm sure I will come up with three better tracks soon, but until then, here are mine so far:
Fatboy Slim - The Rockafeller Skank (http://www.youtube.com/watch?v=hndsFLxjldU)
I think this is sampling in perfection. It's also one of the few tracks that was over the last 11 years played a million times in all sorts of movies, commercials, apre ski parties, high school dances and commercial radio stations, but still has enough credibility to make the dancefloor burn, whenever a DJ drops it. (And it's one of the few good memories that I have about a field trip to Berlin. Berlin is SO overrated.)
Josh Wink - Higher State Of Conciousness (http://www.youtube.com/watch?v=0f7cylz6tl4)
This is probably the most sampled track EVER! I could make an hourlong mixtape only with tracks, that use this sample! But I don't judge anybody who is using it for his own tracks, because it's such an insane bomb.
Underworld - Moaner (http://www.youtube.com/watch?v=wOlTK7q6f1k)
Wait a second, don't you mean "Born Slippy"? Well, I'm not saying that "Born Slippy" is bad, but "Moaner" has so much more energy and is, despite being played live on every Underworld concert, lesser known. So it fits the title of the thread (Songs everyone SHOULD know) better. Did you know it was on the soundtrack of "Batman & Robin"? I'm not kidding! They are playing it in the background during the motorcycle race and even released it as single! Seriously, I got it here! It has a photo of Batman and Robin on the cover and features a crazy, 10 minutes long remix named "Relentless Legs Mix".