Just a discussion on what is the most covered song ever... I'd say it's Led Zeppelin's Immigrant Song, there is literally a version for every style:

Just a few I found on quick youtube search:
Demons & Wizards' version: https://www.youtube.com/watch?v=yP77LfakaFY
Infectious Grooves: https://www.youtube.com/watch?v=eORgaojWu6o
Gotthard: https://www.youtube.com/watch?v=f9RLk48rLZY
Sebastian Bach: https://www.youtube.com/watch?v=1zIRLk1mfU4
Adagio: https://www.youtube.com/watch?v=917efsAE7Cc
Heidevolk: https://www.youtube.com/watch?v=W1vD8-O7CKk
Stryper: https://www.youtube.com/watch?v=VeA3uebJb7M
Enslaved: https://www.youtube.com/watch?v=PBISfxoORj0

And the list goes on...

What do you guys think?

You forgot about Torrent Razor version for 'A Girl With a Dragon Tattoo'

I'm guessing Hallelujah by Leonard Cohen. It's not just metal bands which cover. Summertime Blues is also heavily covered.

I'm guessing it's Happy Birthday.

Very likely

"Sunshine Of Your Love" might be covered way too much. "Enter Sandman" or "Unforgiven" as well. It depends. What types of covers are you talking about? There's an overabundance of guitar covers of the same songs over and over and over again...

How about Mony Mony?

Happy birthday for sure.

Smoke on the water. Possibly the most famous rock riff of all time... and the most covered.

I agree. Smoke on the water is probably the most covered https://www.youtube.com/watch?v=WLklq8SVmJw

Enjoy the Silence

What is a cover, exactly? Where does the line go? There are blue and jazz standards, overlapping with Great American Songbook concept, that aren't really called "covers." Traditional songs (you know, Scarborough Fair, Greensleeves, Child Ballads etc.) aren't covers, either. Neither are arrangements and performances of classical music. The idea of "originality" or that certain songs "belong" to certain performers really only applies to popular music from the 50s on or so.

Happy Birthday, for instance, is a traditional song.

I don't think it's possible to name the most-covered song unless you restrict it to published recordings. We have no way of knowing about live performances. Even then, is a YouTube bedroom recording equal to a major label release?

I looked up a few lists (which did not explain their method or sources, though), and it seems Yesterday, Eleanor Rigby, Cry Me a River, Imagine, Satisfaction, Hallelujah, Summertime, Somewhere Over the Rainbow (if you accept these two under modern "covers") frequently appear.

I am less interested in the absolute number of covers than in their variety, quality and success, though. It would be cool if you could link to the relevant covers of the songs you suggest so we get to hear them.