Personally I feel that some of the most Epic material is Power Metal. Blind Guardian, Adagio and of course the masters, Rhapsody, come to mind as having released some of the most Epic material ever. Rhapsody has it all, intense build ups, good versus evil theme, and a marching into battle atmosphere.
As wonderful as Opeth is though, I would have to agree with others and say they are not wholly epic. While they have their moments, it isn't enough to maintain that Epic feeling, especially when compared to Power Metal bands. Those crazy Power Metal bands are Epic maniacs.