For normal ambient, Biosphere - Substrata is absolutely essential and one of the absolute best albums in the genre as far as I'm concerned.
Also, Tim Hecker is great. You can pick almost any album from his discography and it will be great.
And don't forget Brian Eno's ambient stuff. Hugely influential.
When it comes to dark ambient, Lustmord - Heresy is essential. I don't care if people say it's not the best. It set the standard for all subsequent dark ambient and it holds up better than virtually all other dark ambient I've heard. It's the one dark ambient album that I still listen to sort of regularly.
Troum is good too, but there are so many Troum releases that I can't keep track of them.
Also, Tangerine Dream - Zeit had a big influence on dark ambient and is a worthy album in its own right (Don't get me started on their other albums; I love Tangerine Dream.)
I generally don't like the ambient stuff that black metal bands do, except if it's just a short part of an otherwise metal album. I find that full-length ambient albums by these acts tend to be pretty mediocre by the standards of the genre. It almost always feels half-assed and like they don't quite know what they're doing. Just because they're great at black metal doesn't mean they'll be great at some seemingly related genre. I have no interest in Burzum's ambient stuff, even though I love some of his metal stuff. Wardruna, on the other hand, are great. But they've always been a full-time non-metal act. It's clear that a lot of effort and care have gone into the particular thing they do.
