Watch the ball, watch the ball, watch the ball.
Then again, I only fly VFR and lack a vertical stabilizer.
The opinions and statements of this poster are largely based on facts and portray a possible version of the actual events.
After a short time, you should -- in VMC -- FEEL when an airplane is uncoordinated. Or, like our glider pilot friends, you can tape a piece of yarn to the windscreen.
(I still think the best answer so far has been the Garmin 3X!)
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.