We could wrangle about this all day. In fact, we probably will!  

 I'd recommend:

Voltmeter
Oil Pressure
Water Pressure

Because the water boxer has so much plumbing, having a water pressure gauge
will immediately tell you to start looking for a leak before it becomes
serious and may actually be the most useful gauge of all.

Even better, mount a digitool (see
http://scribers.midwest.net/midwesty/vw)on the heater cover with velcro,
which includes a voltmeter, and go with gauges atop the dash for oil
pressure, water pressure, and fuel pressure.  I'd rank these above vacuum,
ammeter, and oil temperature for waterboxers.  Oil temp is a must for
air-cooled, but maybe redundant on waterboxers.  

Bill
St. Louis, MO
www.geocities.com/wdummitt