I like Brian Eno's Discreet Music [1], which provides three slowed-down variations on Johann Pachelbel's Canon in D Major (the first one rather amusingly named Fullness of Wind), eventually reaching tempos where you can hardly discern the original melody.

Max Richter also brilliantly "recomposed" Vivaldi a few years ago, turning his Four Seasons into something like a modern work.

[1] https://en.wikipedia.org/wiki/Discreet_Music

[2] https://en.wikipedia.org/wiki/Max_Richter#Recomposed_by_Max_...