VDO or Auto Meter are my favs for auxiliary gauges.
Stack for the speedo, but they are pricey.
Oil Temp and Oil Pressure are great reference gauges. They are not necessarily accurate but they do alert you to changes in your system. So my oil temp may normally read 190, but the actual oil temp is 185, but if for some reason my gauge pops up to 220 I know something is up.
I also like a voltage meter, tells you what you alternator/generator is up to.
If you want to get crazy a cylinder head temp gauge is kind of cool as well as air mixture.
VDO Gauges: