Brian De Palma,
Francis & Sofia Coppola,
Shusuke Kaneko, ... I mean I could go on...
The reason why I start with directors is that you can only have an epic soundtrack when your picture is just as epic as the soundtrack. If you have an overly ostentatious soundtrack with a picture that lacks any kind of important element, then this could hinder making an epic movie (which most of the time requires an epic soundtrack that FITS). The director makes or breaks the film's epic-ness as a whole.
Definition of Epic:
heroic; majestic; impressively great; of unusually great size or extent
So on sound side based off of that definition, in my opinion, audio that brings you closer to the hero, spans clarity over an environment creating a majestic soundscape, tells the story with great detail and thought, and sometimes my favorites are enormously spectrum filled monster roars (see World of Warcraft: Cataclysm Trailer)
One of my favorite cinematic openings is "Old Boy":
Quality has been smashed and the impact is softened by no subtitles, especially if you havent seen the movie. But if you have, you rock.
Have you ever tried to design sound to an animation that lacks detail? You can design the most intricate detailed spectrum-interesting filled sound fx for an event, yet if the picture has nothing to visually emit your unbelievable sound design... then its pretty hard to make it epic. HOWEVER, art movies like "koyaanisqatsi" (Scored by Philip Glass) and documentaries like "Rivers and Tides" are in my opinion epic not because of the detail, but of the minimalism involved with the soundtrack that MATCHES the minimalism of the film.
Epic for entertainment, I believe, is a balancing act between picture and sound.
In just audio? Pink Floyd, Mars Volta, Led Zeppelin, King Krimson... on and on and on :)