Here are some pieces I like to play in the background:

Schnittke Symphony 2
It's generally quiet and relaxing, although it has great beautiful dissonant outbursts at times...

Rautavaara Symphony 7
Plain relaxed and beautiful. I might get hate, but as far as the Fins go I prefer him to Sibelius...

Some more Schnittke, Choral
I often put it up in the background myself, it's very soft and soothing, not to mention athmosperic

And Jon Leifs's requiem even more so, it's just about the saddest piece (certainly the most tormented and haunted) I can think of right now.

Not understanding the lyrics, it doesn't sound so sad. Looking them up, it sounds much more sad :P

For sad music, though, I think you'd be hard pressed to beat Samuel Barber. All of his music is basically Music To Kill Yourself To. :/

Reinbert de Leeuw's take on Satie is one of my favourite things:
https://www.youtube.com/watch?v=slahwoLdEIE

Yay for Greek progressive rock!

I like to play this on the background while I do some work on the computer or read
Vangelis - L.A. November 2019 (Blade Runner 2003 Esper Edition)
It doesn't get any more ambient than this!

Great thread! Although already a week old, I just happened to stumble upon it, so I have to contribute:

Since you seem to appreciate the brilliant space-ambient duo of Carbon Based Lifeforms, I reckon you already know this masterpiece by them. If not, have a look at it. It's probably one of my favourite songs.

CBS - MOS 6581 https://www.youtube.com/watch?v=syyl1rKpodw

I can also recommend a lot of the early stuff of Aphex Twin (e.g. Selected Ambient Works). Here a few great examples:

Xtal https://www.youtube.com/watch?v=PnTFKWAT-XE
Polynomial C https://www.youtube.com/watch?v=wpDmZVYC8KU
(In my opinion, CD Project Red should use this one in their upcoming Cyberpunk RPG^^.)
Rhubarb https://www.youtube.com/watch?v=_AWIqXzvX-U
(very soothing)
Stone In Focus https://www.youtube.com/watch?v=goDd02TTxBA
Alberto Balsam https://www.youtube.com/watch?v=ihyQf8mww3o
Heliosphan https://www.youtube.com/watch?v=0Z4cLmbw6q0
Ageispolis https://www.youtube.com/watch?v=Uw7TY-zPaas
Crying In Your Face https://www.youtube.com/watch?v=AhMukHbUYJ0 (less ambient, but had to be included)

Needless to say, this man is obviously a genius and well ahead of his time... Just my humble opinion.

Also for those who prefer instruments rather than electronic ambient I can recommed post-rock/post-metal:

Long Distance Calling - Apparitions https://www.youtube.com/watch?v=3yRBPbtPpS4
Cloudkicker - Let Yourself Be Huge (Album) https://www.youtube.com/watch?v=NiS0vtfSUxM

Rather different category, but also this guy:

Estas Tonne - Song of the Golden Dragon https://www.youtube.com/watch?v=xVZgUiRljdU

Enjoy!

Edit: Oh, and how could I forget...

Final Fantasy VI - Terra https://www.youtube.com/watch?v=QGXMELvSsfc
Orchestrated Version https://www.youtube.com/watch?v=sT9ReWDV0IU

My favorite soundtrack of all time is from the mini-series "The 10th Kingdom". The series was epic, and I think the music is just epic as well. It might be nostalgia, but I absolutely love it. https://www.youtube.com/watch?v=o-xDWZmeuJg&list=PLlW2mt910-7Mb6z-Css2nuFdYSXvxXGYA&index=1

If it's at work, rather than wasting a lot of time in "research", you could take the easy way by subscribing to the streaming radio service "Hearts Of Space". Each program is generally around one hour of music centered around a specific genre or sound. They have recorded broadcastings up to 1983, that means more than 3 decades of recordings and thus it also serves for musical history purposes. Artists and name tracks are mentioned with complete references on each program page so you can later find and buy them if you wish.

Hearts Of Space official website:
http://www.hos.com/