Black Rose Immortal is, I must say, the most obvious answer to this question.
However, I would also give strong consideration to Advent - in a lot of ways it's Opeth's masterpiece track... it really does touch all Opeth bases (up 'till that time, anyways) and provides many memorable instances without being 20 minutes long like BRI. Also, Forest of October and The Twilight is My Robe rank as well.
I think it would have to come from the first two records... MAYH is epic as an album, not as individual songs, and nothing on Still Life or Blackwater Park really reaches that high point of the epic scale, with the exception of perhaps "The Moor".
So with much needless rambling, I basically concur: Black Rose Immortal.