Tachometer, water temp, speedo would be my top three.
My nitrous gauge and airshifter are on their respective bottles, simple to see whats in there when I twist the valve. After 30 years with nitrous, I'm gotten pretty proficient with tuning a nitrous motor, so no a/f meter for me. I have a regulator I've been meaning to put on the nitrous bottle to guard against over pressure, but just haven't got one of them round twoits. Not worried about fuel pressure from either fuel pumps, that's covered with a pressure switch for a just in case scenario.
The gauges I have, I don't really pay a whole lot of attention two after the initial warm up. The water temp will get glanced at a few times after warm up, the oil pressure as well (The warning light is a flashing LED on the instrument cluster and it's wired up to the speedo/tach's oil light for easy seeing if it turns on). Volt meter, same thing. I see it when I first start up the bike, then not too worried about it. I don't even pay attention to the tach that much, I have a rev-limiter and a shift light, so I'm not overly concerned with over-revving or when to shift. Hell for a while I didn't even use a tach, I just had the instrument cluster (oil, water, volts and lights), speedo and a shift light.
One thing I might add, there is a parking brake lamp on the speedo/tach. It's a "P" I was going to wire it up to the nitrous purge, but I got to thinking about brake lights. So I might wire it up so that if/when I loose the brake light, it will illuminate. We all know how much cages pay attention to bikes, running around without a brake or tail light can be a messed up thing.
My number 4 gauge would be my fuel gauge. Didn't think much about it until after I put it on, cause to me it's not a big deal to count mileage. But, after installing it, I loved it.