Well my top 3 would have to be:
Earache Records, they have some of the best Extreme Metal bands out there and produce quality merchandise and music. Notable bands: Decapitated, Cerebral Bore, Napalm Death, Bolt Thrower, Deicide, Carcass, Municipal Waste, Hate Eternal and At The Gates.
Relapse Records, have you listened to Devourment's latest album???? God damn that album has some awesome producing on it! They never disappoint. Notable bands: Devourment, Obscura, Necrophagist, Pig Destroyer, Dying Fetus, Cephalic Carnage, Spawn Of Possession and Revocation.
The Black Wreath, this is a small up and coming Metal label originating in Alice Springs, Australia. They have some of the best upcoming acts out of Australia including Miazma, Uncreation, Exdemission & The Horror. I must say that they hold a higher value to me due to them being my home towns best Metal label.
(this is in no particular order)
(Notable bands may not be currently signed to the label before mentioned due to this being just off the top of my head)
Speaking of Devourment, have you checked out their latest album? It is one of the best albums I've heard this year and Devourment are yet again showing that they are one of the biggest and best bands in Slam/Brutal Death Metal.
Favourite song: http://www.youtube.com/watch?v=jR4lLJu_-wE
Entire album: http://www.youtube.com/watch?v=amG-jBH6mb4