Movies, maybe, but I was thinking about all the sports events that begin with the national anthem. Probably more in one year than all the movies ever made.
But I concede that the Wedding March would be up there, too.
I’d bet the Beer Barrel Polka is played five times as often.
Now that’s really disturbing to watch. 
The word “sampling” suggests that the music is used in a context other than its original intention, which rules out ballgames and weddings and “Happy Birthday” in Real Life full-length uses of those pieces.
Great, because I’m pretty sure that Mendelssohn was writing for the concert hall, and not for Aunt Sadie’s wedding. And the music for “The Star Spangled Banner” was, IIRC, an old drinking song.
I doubt a piece of classical music was the most sampled. Most likely it was a drum break because you can create lots of new songs on top of it.
My vote is for the drum break at 3:30 in “Ashley’s Roachclip” by Chuck Brown and The Soul Searchers. It was used in lots of hits in the late 80s.This page has a partial listing: Ashley's Roachclip - Wikipedia
Jragon asked about the sense of “sampled” meant, but mentioning the Overture makes it clear OscarGold isn’t thinking of breaks. (I mentioned “Funky Drummer” as a small joke.) Consider the question in terms of excerpting and referencing. cmyk has given several good contenders in this sense.
It seems like OscarGold starts threads but does not return to them.
If we go with that interpretation of the question, then I agree with Zebra that it is probably Pachelbel’s Canon. This is extraordinary: Pachelbel Rant - YouTube
I heard it was the music that played in his favorite strip bar.
Tiddy-Rump,
Tiddy-Rump,
Tiddy-Rump,Rump, Rump.
After 2001: A Space Odyssey, every other commercial on TV featured cars, disposable razors, tampons, etc slowly revolving to Thus Spake Zarathustra.
How to tell when someone has you on ‘ignore’. 
Here’s an admirably complete listing of them:
That’s the best! Time to post that to Facebook…
I’ve heard that the definition of “Class” is when you can listen to the “William Tell Overture” and not think of the Lone Ranger… 