Depends on if you want singing or not...
If you want singing, I'd recommend anything by Loreena McKennit or Clannad, maybe even Enya's Celts album.
No singing, go for the LotR soundtrack, the Last of the Mohicans soundtrack, Dances with Wolves, Robin Hood Prince of Thieves, Conan, Gladiator, Glory, Braveheart, Princess Bride, Amadeus, any Star Wars....
And one of my personal favorites, the Matrix! (Of course, you have to be in a certain mood for this
.
Seriously, I do a lot of writing to music, and LotM, Loreena McKennitt, Glory, and LotR are a few of my favorites.