Easily Death for me. They aren't so obscure that nobody has ever heard of them but in terms of metal they are super underrated imo when you compare them to Slayer, Megadeth, even Anthrax to a degree. Which I find criminal because Chuck was the superior musician who never really made a bad album. Everything was solid and each had their unique sound. I have them above Slayer (R.I.B is a masterpiece though.) Leprosy, Spiritual Healing, Human, Symbolic just to list a few of my top favorites.
It took me till I was in my early 20's to even discover the band right before Chucks death. I was blown away with his guitar playing and where the fuck they were all my life. Greatest DM band of all time yet a lot more people have heard of Morbid Angel and Cannibal Corpse. Aside from Altars of Madness, they blow them both out of the water. Probably the most consistent metal band I've ever heard besides Sabbath, but they were more of the bridge between rock and metal. Sabbath is just Sabbath. Noone sounds remotely like them.