I'm gonna have to go with The Olden Domain and Quinessence. The Olden Domain had that really cool organic and atmospheric feel to it. The playing was a little looser on that album, not so precise which gave Borknagar a unique edge. Not to mention how creative Oystein's riffing style is on that album. Garm falters a bit when his vocals go into the lower register but his performance is still creative and inspired.
Quintessence is also really good. It's a bit different for the Bork. At times it sounds a little spacey to my ears. Particularly impressive is Vortex' vocal performance. Just listen to "The Prescence is Ominous", "Colossus", or "Revolt" for proof of this. The music is masterfully constructed with every instrument taking an equally important role, not a common feature in a lot of metal. Oystein once again proves on this album that he is one of the most creative guitarists in metal, utilizing the instrument to it's highest textural capabilities.