What is most important of the five virtues on Gawain's shield?

Valor

"...the emblem of the Pentangle, the five-pointed star."

it is truth....

The poet pays particular attention to the Pentangle, the emblem of truth, known everywhere as "the endless knot." It is particularly suitable for Gawain because the five points of the star represent the five different ways in which Gawain, like purified gold, embodies faultless virtue.