Overkill - The Years of Decay and Horrorscope are essential, WFO, Killbox 13, Bloodletting, and Necroshine are good, I Hear Black is ok, the rest are 'meh' to me
Destruction - never listened to them
Annihilator - Never, Neverland for me, but Alice in Hell appeals to more people for some reason. I also really liked Carnival Diablos.
Kreator - Coma of Souls is their best, IMO. I should get that one again. Renewal was really cool, but waaaay different (more industrial than anything).
Sodom - never listened to them
Artillery - never listened to them
Coroner - Mental Vortex is the best, IMO. John Frank would shoot me for saying so, but I don't think they're essential.
Exodus - I'm an Impact is Imminent fan, but most people dig their earlier stuff better. Anything after Impact is a throwaway IMO.
Metal Church - the debut is the only one I've considered getting, but I haven't heard them all
Toxic Holocaust - never listened to them
Nuclear Assault - Handle With Care and Out of Order are great, but probably not essential. Their earlier stuff was hit and miss for me, but definitely some great tunes in there
Sadus - I personally thought they got better on each release, excepting the brand new one. Many prefer Swallowed in Black or A Vision of Misery.
Vio-Lence - can't stand the vocals on the debut, but Opressing the Masses is definitely essential thrash. the last one was weak
Sacred Reich - Ignorance, Surf Nicaragua, and The American Way are all essential. The two after that are not thrash, but Independent is still good music (Heal is not so much).