Mostly Drop D, but some of their newer stuff from Wrath and Resolution is in Drop C#. For example, Reclamation from Wrath is in Drop C#, along with most of the Resolution album. Not all of Wrath is though, I know that In Your Words is definitely Drop D. Apart from that, pretty much everything with the exception of Vigil (which is in double Drop D) is in Drop D as far as I know.
If you're looking for some cool riffage to learn, I recommend In Your Words if you think you're up for it. Great fun to play because of the tempo and movement up and down the fretboard.
Also, it should say what the tuning is on the tab. I recommend Songsterr as the tabs are generally pretty accurate and there's an online player too.