2004 is looking to be fantastic...
OK, thus far for top contenders (in no order):
Wolverine - Cold Light of Day
Megadeth - System has Failed
Drudkh - Autumn Aurora
Into Eternity - Buried in Oblivion
Woods of Ypres - Pursuit & Allure...
Orphaned Land - Mabool
Mars de Grises - The Tatterdemallion Express
Jag Panzer - Casting the Stones
Arsis - A Celebration of Guilt
Void of Silence - Human Antithesis
Other great releases that may or may not end up in this leading list:
Runemagick, Death Angel, Exodus, Hate Forest, Deathspell Omega, Magyar Posse, Pelican, Iced Earth (the second disc), Blut Aus Nord (although this should be 2003 I guess).
I can't judge Amon Amarth or the new Vader until I've heard them in their entirety, but so far pretty good...
...and I'd really like to hear the new Mastodon.
Slough Feg I guess won't release anything until next year, which is of course when the Hammers release their next album, so I'm already salivating at the knob for 2005.
Still, 2004 is looking great, but I agree with Adrian (and others) that there is no single release that seems to be unequivocally head and shoulders above the rest....and as history always show, it probably won't come my way until next year.