The best one I've ever heard was AlgoMusic[0] on the Amiga by Thomas Schürger. I used to spend hours listening to it. And it was a pleasant surprise while searching for it now, to find out that the author has a brand new project called SoundHelix[1] that has all the awesomeness of AlgoMusic, and sounds even more incredible. You should listen to the examples[2].
[0]: http://aminet.net/package/mus/misc/AlgoMusic2_4