I have to agree w/ Aglarion, I can't possibly choose just one, can I?
Here's who I like from each label, and I guess I'll choose 1 at the end.
Ars Metalli: Arcane Sun, Helheim, Lunar Aurora, Menhir
Avantgarde: Katatonia and maybe a bunch of others I haven't heard yet
Candlelight: Emperor (RIP), Insomnium, Myrkskog, Opeth
Century Media: Arch Enemy, Blind Guardian, Borknagar, Dark Tranquillity, Finntroll, Iced Earth (er, no longer, right?), Kalmah, Marduk, Naglfar, Nevermore, Nocturnal Rites, Old Man's Child, Sentenced, Shadows Fall, Sonata Arctica, Suidakra, and probably more...
Earache: Decapitated, The Haunted, Morbid Angel
Hammerheart: Ancient Rites, Cruachan, Primordial, Skyfire, Thyrfing
Hevy Devy: Buzzzz, sorry!
Metal Blade: Amon Amarth, Ancient Rites, Bludgeon, Bolt Thrower, Brainstorm, Callenish Circle, Chinchilla, Jacobs Dream, King Diamond, Labyrinth, Manowar, Riot, Six Feet Under, The Crown, Vader, and maybe a few more...
Misanthropy: I'm afraid I don't know who was on them...
Moonfog: Darkthrone, Satyricon and possibly 1 or 2 more
Napalm: Abigor, Falkenbach, Hollenthon, Odhinn, Otyg (RIP), Sirenia, Summoning, Svartahrid, Vintersorg
Necropolis: Dark Funeral, Enslaved, Witchery
Nocturnal Art: none
Nuclear Blast: Agathodaimon, Bal Sagoth, Darkane, Dissection, Hypocrisy, Immortal, In Flames, Meshuggah, Sinergy, Soilwork, Steel Prophet, Susperia, Tidfall
Osmose: Enslaved and possibly a few more
Peaceville: none
Relapse: Dying Fetus, Nile, Origin
The End Records: Agalloch, Arcturus, and probably a few more I haven't heard yet...
Falling under the heading, "Other," we have
Dark Symphonies: Novembers Doom, and maybe a couple more
Limb Music/SPV/Steamhammer: Concerto Moon, Human Fortress, Mob Rules, Montany, Rhapsody, Freedom Call, Judas Priest, Kreator, Overkill, Savatage, and probably a few more
Massacre: Hyperion, Metalium, Night In Gales, Pretty Maids, Skullview, Stormwind
Noise/Sanctuary: Bruce Dickinson, Gamma Ray, Grave Digger, Heavenly, Helloween, Kamelot, Queensryche, Running Wild, Skyclad, Virgin Steele, W.A.S.P.
Olympic: Absu and maybe a couple more
Spinefarm/Universal: Arise, Charon, Children Of Bodom, Ensiferum, Eternal Tears Of Sorrow, Kalmah, Moonsorrow, Nightwish, Norther, Throne Of Chaos
World War III: Aborym, Dawn Of Dreams (if they still exist), Thy Primordial
Shit, this is definitely an impossible call, but it looks like I'm going to have to go with Century Media, as much as it pains me. Spinefarm, Hammerheart, Nuclear Blast & Napalm all come in a close 2nd.