wideband, oil pressure, oil temp, and an electronic boost gauge..
though i have meth/flow pressure, wideband, oil temp, and an e boost gauge.
meth gauge will tell me if theres an issue with that system,
wideband will tell me if something fucks up fuel related as well,
oil temp will warn me of various mechanical issues, or how hot the oil in my system is for shutting down.
an accurate boost gauge will let you know if there's a boost issue.
an oil pressure gauge will let you know if theres a problem in relation to oil delivery, but often times i'd figure it wouldn't give you enough notice.. which is why i didn't add one in to my car.