A special "all non metal favourites" selection of mine, again, not all amazing musicians to some, but I like them enough to be added:
Brian Eno - Before And After Science
Radiohead - The Bends (again, since they aren't metal...)
Faithless - Reverence
Gonga - From Under The Trees
Leftfield - Leftism
Lazyboy - Lazyboy TV
Madness - Divine Madness
Mortiis - Født Til Å Herske
Pitchshifter -
www.pitchshifter.com
Vangelis - Odyssey - The Definitive Collection (because its got a bit of everything on it...)
If I wasn't into metal too I'd be fighting myself over these bands and albums, but choosing from mostly metal bands is a lot easier, since in the list above I still couldn't mention The Cure, Daft Punk, The Future Sound Of London, The Prodigy, Basement Jaxx, The Beasties, Bjork

and Massive Attack off the top of my head who have released some of the music that I have most listened to growing up, just metal is a lot easier to pick 10 or so that I really enjoy
Still, out of Opeths catalogue I would say Morningrise, and they have a bit of everything so I don't mind calling that my overall favourite album, and yeah it happens to be a metal release.