Limit it to your one best, as I'm sure we could all list twenty with no problem.

Mine is Aphex Twin's ".215061" from Analogue Bubblebath 3:

http://www.youtube.com/watch?v=HFYOiwH9EgY

http://www.discogs.com/AFX-Analogue-Bubblebath-3/release/28763

Cheers, sneak