when it comes to Instrumental, http://www.youtube.com/user/AdrianvonZiegler/videos is king
He does several different Genres, his album "Requiem" has some pretty epic tracks (Dein Licht in particular)
and then there is this remix of the http://www.youtube.com/watch?v=q-_YhTj9hU0 thats sorta epic
https://www.youtube.com/watch?v=MtN1YnoL46Q
I'm not going to lie.. This is pretty epic.
Can't get any more epic than Tengen Toppen Gurren Lagann. Which is an anime. Seriously. I think it has one of the most powerful beings and epic fight scenes ever devised in it. Some of the music can be suitably epic. I recommend you watch that anime. Even if your not an anime type it must be seen. It's up there with the likes of Cowboy Bebop as far as I'm concerned. For epic music now? The Vision of Escaflowne has some good music. http://www.youtube.com/watch?v=O_49H_N43jQ, http://www.youtube.com/watch?v=YNz2RnH6ClE, http://www.youtube.com/watch?v=ejyBcX_gUNQ, and http://www.youtube.com/watch?v=ydo7Y4CljYc. I can also recommend the series.
Edit
Also I found this nice piece. http://www.youtube.com/watch?v=WwOKKU61jh0. I used it as a death theme for the space marine deathwatch kill team in my 40k game I host. http://www.youtube.com/watch?v=Miogt0wzHD4 which I used for a defense against Necrons theme. http://www.youtube.com/watch?v=a_mm3eI2qHQ another for the death of a kill team member. http://www.youtube.com/watch?v=UkJi4ptY9BA I used for one of my party members going insane and slowly falling to chaos. http://www.youtube.com/watch?v=GtWrUGdDU6M theme song. It should be obvious. I have Gods Of War in my signature. http://www.youtube.com/watch?v=L_QP9qYaOq0, I used for the kill team in general and for fights. Space Marine launch trailer music http://www.youtube.com/watch?v=Obj-aHG-Ix4 is also good.
Finally I have http://www.youtube.com/watch?v=uDjZceUNr3E from the upcoming 300 sequel.
This is all but a sampling of some of the epic music I like and have curated for various efforts.
Edit2
Not sure what song http://www.youtube.com/watch?v=8JR2CU4ApCI is (1 minute mark), but I use it for my Vostroyan Imperial Guard game I'm running.