I consider myself pretty well-versed in melodeath from 20+ years of listening, but I'm sure there are some essential albums I've missed. I wonder if it would be useful to build a list made up of the favorites of members who wish to participate. I'm not really interested in "above-average" or "very good" albums, only the absolute classics.

Maybe we can do this by posting your top 1, 2 or 3 of the genre, although if someone has listed an album don't list it again.

My favorite melodic death metal albums are:

1 - Carcass - Heartwork
2 - Edge of Sanity - Crimson
3 - Intestine Baalism - An Anatomy of the Beast
4 - At the Gates - Slaughter of the Soul
5 - In Flames - The Jester Race
6 - Sentenced - Shadows of Past
7 - Arghoslent - Hornest of the Pogrom
8 - Hypocrisy - Abducted
9 - Sacrilege - The Fifth Season
10 - Dark Tranquility - Fiction

At The Gates - Slaughter Of A Soul
Dark Tranquility - The Gallery
In Flames - The Jester's Race
Kalmah - Swamplord
Soilwork - Predator's Portrait
Eternal Tears Of Sorrow - A Virgin and a Whore
Sentenced - Amok

just to name a few...there are lots of hidden gems of bands that play highly quality melodeath, try looking for similar bands on this page

I've been having recurrent bouts of intense infatuation with these albums for a quarter century now:

Sentenced - North from here (probably the absolute best of all melodeath albums in my estimation)
Hypocrite - Into the halls of the blind
Mörk Gryning - Tusen ar har gatt
Vinterland - Welcome my last chapter
Dawn - Naer solen gar niper for evogher
Eucharist - A velvet creation, Mirrorworlds
At the Gates - The red in the sky is ours, With fear I kiss the burning darkness

Here are my nominations:

1. Insomnium - Since the Day It All Came Down
2. Dark Tranquillity - Damage Done
3. In Flames - Whoracle
4. Omnium Gatherum - New World Shadows
5. Mors Principium Est - The Unborn
6. Amon Amarth - Surtur Rising
7. Noumena - Absence
8. Opeth - Morningrise
9. Nightrage - Sweet Vengeance
10. Scar Symmetry - Holographic Universe

And here are some bonus extras, in no particular order (including some outlier choices that might not appeal so much to others, but that I love):

The Wake - Ode to my Misery
Arch Enemy - Burning Bridges
Shadow - Shadow
Words of Farewell - Immersion
Brimstone - Carving a Crimson Career
Soilwork - Natural Born Chaos

God Dethroned - The Christhunt, The Grand Grimoire, Ravenous, The Lair Of the White Worm
Excretion - Voice Of Harmony
Detonation - An Epic Defiance, Portals To Uphobia
Gates Of Ishtar - The Dawn of Flames
In Thy Dreams - The Gate Of Pleasure
Mors Principium Est - Inhumanity
Convulse - Reflections
Desaster - A Touch Of Medieval Darkness
Morna - A Tale Of Woe

Amoral - Reptile Ride
Anata - The Infernal Depths of Hatred (later stuff is techier but this certainly fits)
Garden of Shadows - Oracle Moon
Unanimated - Ancient God of Evil
Unanimated - In the Forest of the Dreaming Dead

+1 God Dethroned, Gates of Ishtar, Edge of Sanity, Sacrilege