Mark Mancina's “Pink Panther” theme, played as a fugue in the style of J. S. Bach, by Stéphane Delplace
Youtube user grantwoolard took 57 classical pieces, from 33 different compositor (from JS Bach to John Willams), and created this six minutes mashup, with an animated sheet music.
Mighty Mike is a (french) user on Soundcloud, who creates mashups between music that shouldn't mix well. And yet, it works. Here are a couple of my favorites