I can't believe nobody has put up Firesoul yet.
This is a really hard thing to boil down. I have to pick While Heaven Wept The Furthest Shore. Its long, its epic, it hits you in the feels, it has hands down THE best album artwork ever released. I think if I had a few more picks this might not be on it, but this fits so many of the things that make metal amazing into it that I think its a good choice.