I have a few of them already, and I might download the entire discography, so I hate to miss the best album(s). From what I've heard, "Blood Fire Death" and "Twilight Of The Gods" are great CDs. Any others?
Under the Sign of the Black Mark is a must-buy, and The Return isn't far behind. Blood, Fire, Death is arguably their best, though I tend to favour UTS...
Though I'm a huge black metal fan, I like Bathory's viking stuff a lot more.
I'd say Hammerheart, Nordland I, Blood Fire Death, in that order. The others, I haven't given much of a chance to yet.
I'll probably get bashed for this but, I'm gonna have to say Nordland 1. I should also mention that I have heard everything else from Bathory and this is still probably my favorite for some reason.
It's all a matter of taste, so you are going to get tons of different answers. Best advice I could give you is to check them all out or the best ones from each era of Bathory to get a feel for the band.