Yep, Kind of Blue...I can listen to it 1000 times and there are still some things that sound/come across different.   It probably is the most important Jazz recording of the last century, at least in terms of opening up the "new era" of Jazz.  A classic, and I've seen it recommended as a must listen especially for non-Jazzbos to get an introduction to the modern jazz sound.  The reason the improv thing was so open on that recording is because most of the tunes were modal, not standard chord vamps in the II/V7/I  sense.  I heard that Miles basically told 'em something like...OK, just play some shit in Lydian, Mixolydian whatever and they picked a groove and went from there. Very little structure, just a few bars of the head were written.   Actually most jazz now is like that, unless it's a Standards session or Big Band.  Three bars on paper, then you're on your own.