I will not rank releases relative to each other with any kind of rating system, I will just identify which albums were essential listens in my mind and which were valiant efforts that deserve an honorable mention but fell short for one reason or another.
Horrendous - The Chills
Necrovation - Necrovation
Ataraxy - Revelations of the Ethereal
There were certainly other cool releases in 2012 like Drawn and Quartered - Feeding Hell's Furnace, Desolate Shrine - The Sanctum of Human Darkness, Ignivomous - Contragenesis, Wrathprayer - The Sun of Moloch, among others, but I think these were the absolute best and the releases that will leave the largest marks on the genre moving forward.
Also there were some great releases outside the narrow confines of OSDM; in terms of hip-hop El-P's Cancer 4 Cure was an album I thoroughly enjoyed all year. Check that shit out.