Moriarty's theme, heard the times he pops up randomly, and in scenes of tension, so...most of the time he's on screen, then. As evidenced here.

And then there's the famous ringtone, and the 'Thieving Magpie' track he stole the Crown Jewels to, which I've linked in a bunch of things already.

My personal soundtrack has added Taylor Swift's Blank Space, as well as a whole bunch of others but I'm in a rush now so will maybe link later. :)