
Oil Pressure, Oil Temperature
Coolant Temperature
Manifold Vacuum
Ammeter, Voltmeter

Fuel Pressure [maybe] (Problematic, since you either need an electronic
sensor/gauge pair or you have to mount the damn thing outside the car)

In addition, it'd be nice to have a big red idiot light 'Check Guages'
connected to Oil pressure, Oil Temp, Coolant Temp, Ammeter &
Voltmeter.  With heaps of guages, it's hard to look at them all all
the time.  In the case of oil pressure, for example, you want to know
right away if your oil pump goes bad, unlike coolant temperature, a
minute or two of 0 oil pressure would be A Very Bad Thing(tm).

Adam