My priority:
1) oil pressure (this is the absolute most important gauge for a performance car and the Z34 don't come w/one stock! LOL!)
2) B00000st (duh! Of coarse logging the OEM MAP sensor is also a good idea. OEM turbo cars usually have these on the IM)
3) EGT (if you got a v-block I would recommend at least two, one for ea bank)
4) oil temp (if you have this, no need for a "turbo timer" since you can just watch your oil temp drop while parked)
5) water temp
6) fuel pressure
A/F ratio
Why would you want to watch a gauge needle bounce back and forth wildly, or numbers scramble if digital, while driving? Its only gonna hold still on WOT runs and there's no way you're gonna be watching that when you gotta keep your eye on the rd and tach. A/F ratio is only important to me when I'm street tuning / datalogg'n WOT.