I purchased Remission, Leviathan, and Blood Mountain, all right around the time they came out. I think all are worthy albums, but Leviathan is the best by quite a bit.
Their older stuff tends to be less progressive.From the stuff I've heard I'd say Blood Mountain. It has aggression but just enough progression to keep the songs interesting.
I'd start with Leviathan (the one that made me a fan of the band). If you enjoy these, get Blood Mountain and the couple last releases. They're interesting in my opinion.