Unfortunately, the one at Lyrics to the Hymn of the Soviet Union Translated! | PriusChat is dead. It intentionally misheard the Soviet anthem as "I need an enema" + a whole bunch of other funny stuff. is a good one I just learned of (even though it's apparently 5+ years old). I believe the music is from ep 1 of Star Wars (Phantom Menace). Corn on the cob... cola... grilled kabob.. soda... Xena and so on. I wonder what John Williams thinks of it.
He's never seen Star Wars, good luck getting him to watch a Youtube. (yes, sounds crazy right? He claims he's only ever seen the scoring reels, which is not at ALL like the finished films) Neat clip. Reminds me of the Bad Lip Reading guys.
This has long been my favourite of such things. It does help if you know the original. (And those of us who had two young girls when Frozen came out know it perfectly.)
My favourite one was from the early days of YouTube. Some of you may remember "Benny Lava" which is a song in ... Tamil (I think?). Someone made a video of what it sounds like in English (nsfw language... just two instances)