Haven't really just got a favourite, more like a bunch of them ..
Corrosion of Conformity - Stone Breaker, World on Fire, Paranoid Opioid
Down - Temptations Wings, Lifer, Hail The Leaf
Chokehold - Hate Department
Celtic Frost - Synagoga Satanae, Progeny
Slayer - War Ensemble, Raining Blood, Silent Scream
Machine Head - Davidian, The Blood The Sweat The Tears, Old
Decapitated - Flash B(l)ack, Spheres of Madness, The Negation, 69 Days, A Poem About An Old Prison Man
Carcass - Corporal Jigsore Quandary, Incarnated Solvent Abuse, Blind Bleeding The Blind, R**k The Vote
Meshuggah - I, Dehumanization, Soul Burn
Pantera - Anything and everything they've released
Dream Theater - Fatal Tragedy, As I Am, This Dying Soul
It's all good, but those songs are the ones that are sticking out for me at the moment. For you old school Sabbath/hard rock fans, I cannot recommend enough that you give both Down and C.O.C a listen.