Boost is definitely the least important. You can FEEL the effects of turbo operation. You can't feel transmission fluid temperature or fuel pressure or exhaust gas temperature by the seat of your pants. That doesn't mean that you ought not to occasionally check the boost out...if you don't have a stock gauge (and if you do this is actually one of the few stock gauges that actually functions accurately and precisely...kind of like the speedometer, tachometer, and fuel gauge). If you have a stock gauge then you definitely don't need an analog boost gauge. You absolutely need to monitor: Engine Oil Temperature (EOT), Engine Coolant Temeprature (ECT), and Fuel Pressure (FP). It is good to check Transmission Fluid Temperature (TFT) and Exhaust Gas Temperature (EGT) especially for those who tow. It is nice to check Intake Air Temperature (IAT) at #1 and #2 but more important to check the FICM voltage...
If you are looking to replace a gauge the boost is a safe one to select.