There isn't one "most common" lunar phase. The Moon goes through all of its phases, from New Moon to Full Moon, in a regular cycle that takes about 29.5 days. This means each phase, including the New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, and Waning Crescent, are equally common and appear roughly once a month.

It might seem like the Full Moon is the most common because we often notice it the most, but that's because it's the brightest and most visible.