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.
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
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