personally i would go

oil temp - to know wen u can put ur foot down after a cold start
water temp - more accurate general engine temp than the dash
boost - for obvious reasons

maybe air/fuel if u have tuned the car

i would have also suggested oil pressure because my last car was integra type-r and u need oild pressure before vtec will work ... but i dont know how important it is in the wrx as an indication of anything that u can actually do anything about lol

how many of the above points would something like an apexi neo monitor? could kill all the birds with one stone there ...