I vote for a voltmeter, because there's nothing else for this function as OEM in the dash (the charge light doesn't tell you about battery condition.

Engine coolant temperature is more important, but we've got a gauge for that (especially if you pulled the Compensation Board in '86-on cars)

Sure, lot of folks are going to say oil temperature or pressure, but I use a very good synthetic oil, so I don't worry about oil temp. that much (synthetics can take it, and high temperatures would be reflected in higher coolant temperatures, too). And for pressure, when did you last hear of an oil pump failing? Besides, we've got a oil pressure idoit light for really low pressures.

In the real world, it's battery condition that I care about and don't have a clue about in a OEM dash -- you should add one of these first!

If you have the spare space, I'd put a tachometer in after the voltmeter.