Great start on this thread, starting with Tool, 10 000 days.
It is hard to find anything to match that, but there are a few metal albums in the same class.
Third Moon, Sworn Enemy Heaven is one of the select few that qualify IMO. And since the group is relatively unknown, I add them as a very strong recommendation - actually for anyone and not just the metal afficiando.
Third Moon calls from Austria, and they're extremely talented musicians. They have made 5 albums, and the material lack somewhat on several on those. They compensate with absolute impeccable execution.
Sworn enemy heaven is a theme album, and a very dark one as that. If you know metal, think Queensrÿche: Operation Mindcrime on stereoids.
Still here?
All good I tell some more then.
The band Third Moon were founded in 1994, and it took until 1997 before they released 'Grotesque Autumnal Weepings' but it took until the album Bloodforsaken before they had matured to their full potential. Without any break trough for Third Moon several members played in other bands.
Third Moon sometimes is labeled death metal, sometimes black, but the fact is that they transgress the genres and often include one or two tracks that clearly are doom metal as well. It is obvious they have a thorough knowledge of music history, one can hear one almost unshameful reference to Edvard Grieg (1843-1907) in one track, and more subtle references to other classical music geniuses in others.
One other example of their musical prowness this is that they even included a scherzo, and that is a sign that this is intelligent metal. Though perhaps not metal for intellectuals. Many tracks have intermissions with classic Spanish guitar that are excellently played.
That is also true when they swap for electrical guitars, even though I have owned this album for years.
It still happens that when listening with headphones I skip back and still wonder how the heck they manage to do the riffs. Like on Persecution, Sebastian Dominik must have double jointed fingers to do that stuff.
Now that I mentioned headphones, there's a few more things on this album that might have you jump out of your socks. There's one flanged sound with doppler effect that is just amazing, I leave it to you to find out where it is. Also one metal guitar effect that's heard twice that I suspect is sampled, or else they might have spent days of studio work to create it.
But unusually they do use synth on a few tracks of this album so there might have been a sampler around to make their own with, altogether is a very well made production so it is possible they did it from scratch.
But it is only those tree total, but that's why they are so surprising, they do not overdo things with too many effects. Something I consider very smart.
Such would be a mistake, and something that is all too common among some of the nu-metal bands.
So to conclude, I have to quote myself with a phrase I came up with on another forum.
This is metal that gives me visions of madly spinning attractions in a theme park from hell.