It's not something I can listen to all the time, but I have had some great ambient listening experiences from time to time and it would be good to get some input or further recommendations from other listeners here.
Fripp and Eno - Evening star - This could well be the perfect ambient album and I've realized what a long overlooked gem it has been in my collection for many years. The basic idea is to have a continual stream of audio from a live instrument (Fripp) fed through a tape-delay unit (Eno) with the two forces almost playing off of each other. This is further processed and rearranged into layers which phase in and out to form a droning repetitive structure with melody at its core. It's a better album than No Pussyfooting and probably the best project Brian Eno was ever involved in.
Maeror Tri - This group seems more droning and noisy with less emphasis on melodic development, but fairly enjoyable if the mood permits it. Mind you their catalogue is extensive and not so easily penetrable so it would be good if someone could identify a 'best work'.
Lustmord - Heresy - I found this interesting at first but don't find myself coming back to it all the often. It's basically just a 'sound-collage' of sampled audio pitched down and drenched in reverb. It's also kind of lacking in internal motion which is really what drives this type of music if nothing else. The place where black stars hang is better but still doesn't grab me in a big way.
Steve Roach - The magnificent Void - I find this hard to distinguish from The place where black stars hang but I think where this guy really shines is when he is mainly melody-focused. Structures from silence has a warmth and a stillness to it unmatched by any other recording while still retaining the necessary sense of movement or internal motion. While Midnight moon is on a different plain of existence altogether. Deep, dark intoxicating waves of spacious organic sound in seemingly endless repetition. I'm almost forced to slow my heart and mind down while appreciating this recording.
Klaus Schulze - To me he was the master of creating a sense of motion/movement/momentum within his music and shaping it furthermore with a genius tact for tension and release. This is best exemplified in early works such as Timewind and Irrlicht. I still like Moondawn but it's an obvious decline from there as everything starts to sound a bit glam-rock-ish in its choice of sounds/timbres and it's also a step away from ambient compositionally.
Tangerine Dream - People praise Phaedra and Rubycon and won't deny that they're brilliant but at this point I honestly find more longevity in their pre-breakthrough album Zeit. Perhaps it is that I listened to that other stuff a thousand too many times and that Zeit by its very nature is not as immediately accessible, but really I just love the heaviness and vastness of it. Of course they didn't yet make use of any sequencers thus the emphasis was more on creating layers of heavily oscillating sound. There is not so much a sense of movement or tension, but inversely, the lack of it as for over an hour it holds you fixated without doing anything. Interestingly the moog modular synth which comes in at the tail end of the first track is being played by Florian Fricke of the group Popol Vuh, he later sold that same synth to Klaus Schulze who was at one stage also a member of Tangerine Dream.
Popol Vuh - Their first two albums fall partly into ambient/noise and partly into world music, but are notable for the dense atmosphere of 'tone sounds' and interesting array of contrasts. In den Gärten Pharaos is particularly awe-inspiring at times in its ability to convey a merging of the senses; as though one were observing the audial form of the great european frescoes.
Eberhard Schoener - Meditations - Interesting, a good companion to the above mentioned but not overly enduring . This is about as close as Krautrock ever got to something purely droning and meditative, devoid of rock.
Biosphere - Substrata is a genius album but the rest of Jensen's catalogue is largely unremarkable. Why is this? Basically because he was transitional at the time and still retained something of the energy and directness of his roots in house/techno music. Thus he stumbled onto greatness with substrata but failed when the design became codified. The point is that ambient is not slow and quiet, it is pure force simplified to the point of being immaculate. There is no narrative structure but its design is something like a byproduct or afterglow of a narrative structure (and mostly it is that the artist has attempted both).
Time Machines by Coil could be considered the quintessential drone album but it doesn't go for atmosphere and it certainly doesn't acknowledge even the slightest hint of melody. Instead it functions more in the way that beautiful architecture does; revealing inner space rather than outer space. And in a conventional understanding of music appreciation it simply isn't music but it's awesome nonetheless. It's something close to what I think of when the average joe ask me what kind of music I like "well, I prefer noise to music".
Suuri Shamaani is like Time Machines worship but with an even more clinical or sterile approach (a trait oddly enough shared by fellow Finnish techno pioneers, Pan sonic). The project is labeled as 'ambient metamusic' and it is fairly faithful to that description, if you can imagine a picture composed of component parts but no obvious whole