Just depends. Mono used to be popular, and still is in some situations (nymphs), but for the most part people are using either a GSP braided superline (e.g. Spiderwire, Fireline, PowerPro, etc.), coated steel wire (e.g. Beadalon or the AFW equivalent), or uncoated steel wire (e.g. AFW, Cortland Toothy Critter, or Senyo wire). With some of the new shanks, an intermediary material might not even be necessary, because you can run the eye of the rear hook through the front shank (e.g. Fish-Skull Articulated Shank).

What you use is pretty much determined by what you are trying to achieve. Sometimes you are looking for a stiff connection to minimize fouling (e.g. Intruder), sometimes you are looking for flexible connection to maximize motion (e.g. Galloup's Dungeon). If you could be more specific in terms of what you are thinking of tying, a more targeted answer would be possible.