A-list metal

[Eleison]

Would this include just metal, or other genres like prog rock, too?

All works from all genres which meet the standard of, say, Incantation's first album.

That's going to be a very short list, not that I have a problem with that.

[Endless]

And what about electronic/IDM

Kraftwerk
Tangerine Dream
Jean-Michel Jarre
Dead Can Dance
Autechre

add Lord Wind if you really want to put I think Kraftwerk is more imporant.

[Moringotto]

This isn't a challenge, so much as a genuine inquiry.

Would someone explain how Merciless fits into the proto-underground category? 

On the surface, and I haven't listened to them extensively, they don't seem to fit there.

[lost_wanderer]

I'll put Autopsy on the A-list.

Maybe the second album from Dimmu Borgir should be considered too.

Also

rotting Christ
Esoteric

[fallot]

Adramelch (Ita) - Irae Melanox (1988)

The vocals are a definite aesthetic challenge, but this is heavy metal incorporating some elements of death metal/progressive songwriting in a manner similar to, but surpassing, Helstar. A few (what are now) power metal cliches are jarring interruptions, but do not obscure the overall picture. Overall a solidly credible metal album. Thin but effective atmospheric production, has to be very loud to be effective. Wonderful neoclassical music.

http://www.metal-archives.com/reviews/Adramelch/Irae_Melanox/4134/failsafeman

"Take a look at the dark, melancholy painting on the cover, with the castle dimly discernible in the distance; THAT is what the album sounds like, but from a distinctly personal human perspective instead of the misanthropy or epic triumph black metal tends to favor when dealing with similar medieval themes."

http://www.youtube.com/watch?v=OmV2NUviRUQ
http://www.youtube.com/watch?v=Dj6AsLXZEIM

[Barbarossa]

The list up top is really solid.  I do agree on a few additions and the few subtractions I can think of I probably don't know well enough to suggest.'

Additions:

Enslaved
Sacramentum
Ildjarn
Atheist
Candlemass
Cathedral

I also suggest the additions of Dark Angel and Bathory as both of those bands created a top quality album that became a foundation of the genre.  Cathedral, I suppose, also fits into the category of a band that only put out one album of such quality.