You came to the right forum, guy.
Well a really amazing one that puts me in a great mood for writing because it's so cloaking and dark is Morton Feldman's Piano and String Quartet. I think it's one of the best classical pieces ever written. More minimalist than ambient but just ****ing listen to it.
Alvin Lucier's I Am Sitting in a Room is wild and gives you a good scope on what ambient music is all about both directly through speaking and in sound.
Since you mentioned Oneohtrix Point Never I'll drop in some Tim Hecker, a vaporwave contemporary of OPN's who's collaborated with him as well.
Brian Eno's Music for Airport's is essential because it's so foundational to the genre and you seriously hear it in everything once you get it memorized. I legit heard this in a valley once.
Popol Vuh is crazy brilliant in the vein of Eno in the vein of really building a world with soundscapes and they are crazy beautiful. Oh, and they also predate Eno.
The Caretaker is ****ing great. His music is about Alzheimer's and the repetition of memory and the fading away of memories and happiness and fear and **** but even outside of that context his music is really great.
I'm a huge fan of Paul Jebanasam's Continuum. It's a massive sounding apocalyptic, climactic film score-esque journey. Rad stuff.
William Basinski's The Disintegration Loops is a piece of tape played over and over as it disintegrates each time it passes over the tape head. It's a classic of the genre and exemplifies the way that we react to loops in that it seems to gain depth before it fades away. He also recorded it on 9/11 in NY as the towers went down so it's pretty much the deepest music ever made, bro.
Be sure to give them all in depth, full listens, preferably with headphones because progression is a big part of the genre. I feel like I barely even clipped the iceberg too.
Linear Mode
