I have a tee-shirt that says "it takes 2 things to fly - Airspeed and money" Since by now the money (or at least some of it) has already been spent, I would have to say airspeed indicator. On reconsidering, however, if I were in IMC, it would have to be the artificial horizon. I did experience a vacuum failure in IMC once and the electric backup artificial horizon became my primary instrument and almost made it a non-issue. It gives you more information than any other SINGLE instrument to maintain coordinated flight.