I agree with Samael being underrated- although Worship Him, and Blood Ritual are rather simplistic. Yet, Ceremony, Eternal, and Rebellion are masterpieces in my book
How about Neurosis? Been around forever, and damn interesting and good.
No one ever mentions Crowbar- by far the best Southern sludge sabbath influenced metal ( by that the US southern)
Thrash-wise, my fav style, I would have to say Exhorder, Dark Angel, and Kreator have never received the attention they deserve- especially Exhorder- damn they were good.