I submit these for your consideration for greatest mashup of all time, which one though?

Marvin Gaye vs. The Pixies - Get It On My Mind https://www.youtube.com/watch?v=fLrzo8MT2VQ

Marvin Gaye vs. The Verve: Ain't That Perculiar / Bitter Sweet Symphony https://www.youtube.com/watch?v=vJ-YXgoIs8k

Classical Music Mashup: https://www.youtube.com/watch?v=DayjwRDcs44

Jefferson Airplane vs. Fatboy Slim - Somebody To Praise https://www.youtube.com/watch?v=VBtI_sUuL58

Not a purely musical mashup, but one of my all-time favorite youtube videos features a mashup song with the beatles, joan jett etc and clips of the 250 top imdb movies in 2 1/2 minutes: https://www.youtube.com/watch?v=U0x9HtYgVqA

I do not want to know how long that took to create.

Helter Skelter mashup is my preferred one over originals: https://www.youtube.com/watch?v=9VhW67Ya8jo

Many WaxAudio mashups are top-tier, such as: Bee Gees + ACDC: https://www.youtube.com/watch?v=OrlWWrSwaB8 Stevie Wonder + Metallica: https://www.youtube.com/watch?v=KyvxHL5hLfE

Technically I think this is the greatest mashup of all time, although it is mashing up preacher and famous tune rather than two separate tunes https://www.youtube.com/watch?v=pdXek5d2ocw

You also didn't bring up Neil Cicierega's discography, which surprises me.