Bathory

I think 'blood fire death' is where they started to become an interesting band.
 
Bathory includes thrash, viking metal, and some old school black metal (The albums progress with the styles). I like just about all the albums even the last ones NORDLAND SAGA 1 and 2. On the last two albums Quorthon has created some cool atmospheres even tho he wasnt the best of singers on those. He was quite good tho at the rugged viking vocals, and the musical journeys he created are timeless.
 
I'd get Hammerheart first. I think it's their best followed by Twilight of the Gods which is almost just as good. Those are my two favorite. If you like rawer/faster stuff, but still somewhat epic, check out Blood Fire Death.