There's a series of "how to politely decline your stepsister's advances" by Ryan Creamer. I watched it with my son on Pornhub. Talk about a bonding moment.
I remember I clicked into one of his videos from the homepage out of sheer curiosity and ended up getting super distracted trying to solve the take-home exercise at the end
Probably not the most watched, but I know it was a meme for a bit that you can listen to King Crimson albums there because Robert Fripp was adamant with taking down any unauthorized uploads on youtube.
Probably the Rammstein sex clip (I mean, it has sex in it but I don't think fapping to it is it's main purpose so I wouldn't count it as porn...
[Edit: The clip on Pornhub is "Pussy" and not "Sex", altho Rammstein also have a song called "Sex".]
There's a song by the band Rammstein called Pussy (not Sex as I previously said that was an error) and their clip was to lewd for YouTube so they posted it on Pornhub.