the self-titled and the olden domain seem to have come from an entirely different band than what borknagar is today. but the catalogue makes sense, as a whole. its quite interesting to see where the vision was when it started, and what it has progressed to.
that being said, i find it exceedingly difficult to name any of them a best effort, they all offer so much, and in such variety. the first two are perhaps the most brutally beautiful, an example being a track from the first album called grimskalle trell: the drumming, garms vocals (and oh, the screams) are all so inexplicably perfect.
there is also much to be said for the archaic course, but i honestly wouldn't know where to start. suffice to say, if you're a borknagar fan, and you don't own all five albums, you're definitely missing the big picture.