What is your favorite Minecraft mod?

If I have to single it down to one mod it is a toss up between tinkers construct, thaumcraft or Buildcraft. I think im going to lean more towards buildcraft though as it gives you a variety of engines and mechanics you can use to help you mine and make life easier in minecraft. I still have yet to get my head around buildcraft and even more so for applied energetics but i dont use them that often.