I don't know if anyone can track this but I suspect that the "most-played" song on the Internet might be "September" by Earth, Wind and Fire. When I'm just randomly browsing the web, I usually don't have to browse very far to come upon a video of that group performing it or some people dancing to it or even someone trying to match it with a cover version.
Part of me doesn't quite grasp the premise of the cover versions because it would take you just as much effort to find the original version that the cover artists are trying to replicate. Which would you rather hear? The Beatles performing one of their songs or four unknown guys doing their best to sound just like the original Beatles recording? But then another part of me admires the skill involved. Here are some folks doing a damn good job of copycatting "September" by Earth, Wind and Fire…
And here's some folks doing an interesting twist on the song that sounds quite different from the original. Take your pick…