your options
boost gauge
fuel pressure
exhaust gas temperature
oil pressure
trans temp
intercooler temp
air/fuel
intake air temp
knock gauge
a clock
boost is a must
fuel is good to spot fuel pump problems, tune fuel pressure.
egt is good to moniter air/fuel ratio, fuel deliver problems.
trans temp good to moniter the trans that will die sometime.
ic temp is fun to watch, helps you want to do IC mods.
Oil pressure is low on the list, but is an important factor into seeing if your engine is going to die from worn bearings.
Air/fuel makes some cool light shows.
Intake air temp is real expensive, but would be nicer then IC temp.
My reccomendations:
boost,
egt or fuel
temp gauge you can switch from trans to ic