Normal, but I'm thinking about raising it up to the highest.
Combat is pretty darn difficult though, you pretty much die in three hits (on normal). But if you play the game stealthily (like you should), it just gets too easy.
It's also kind of a shame that there's a mechanic quite similar to Arkham Asylum/City, where you can see through walls and detect enemies. I pretty much have that on all the time, which doesnt do the beautiful level design any justice.