I'm crazier than all of you.
26x1.75 Vittoria Adventure up front, 26x1.5 WTB Slickasaurus in back, taken offroad on dirt and gravel.
A little dicey in mud and sand. Otherwise fine. This way, I don't have to drive somewhere with a bike on the back of my car.
Usually works best to have the more aggressively treaded tire in front because a rear wheel skid is recoverable, whereas a front wheel skid is generally not. There is more weight on the rear tire, so it can better dig in. And putting a bigger tire up front deadens the vibrations better.