wideband air/fuel if you really want to know what's going on.. I don't have much faith in the narrow band a/f gauges, but it's
your car. As far as what kind, that is all personal preference. If you're going with any kind of "theme" (blue lights, green lights etc..)
then autometer probably makes a gauge for you. I bought the UltraLite 2's and put green LEDs in them, and it is pretty sweet.