Hmmm, God, this is SO hard to do off the top of my head.
Now, I'm going to honestly list the first 5 metal albums that spring to my mind as personal favourites. I will probably forget some better records while doing so. I just know I'm gonna get flamed...
Opeth - 'Blackwater Park'
Arcturus - 'La Masquarade Infernale'
Dream Theater - 'Train Of Thought' (Yes, I know everyone hates this album, and most people hate Dream Theater. It's true that James LaBrie is a cockface, but I dunno... there's just something about this album that really appeals to me. I listen to it frequently, and enjoy it very much.)
Strapping Young Lad - 'Alien' (Again, another questionnable inclusion, but I just really enjoy this record also. I listen to it all the time.)
Metallica - 'Master Of Puppets'
Honourable mentions:
Katatonia - 'The Great Cold Distance'
My Dying Bride - 'The Dreadful Hours'
Emperor - 'Prometheus: The Discipline Of Fire And Demise'
Akercocke - 'Words That Go Unspoken, Deeds That Go Undone'
Opeth - 'My Arms, Your Hearse'
There... this is FAR too hard