I think a defining moment for the Scooby Gang is in Season 4's Primeval - when they invoke the powers of the Slayer lineage and the enjoining spell kicks in: they merge their psyches in Buffy's body comprising of Buffy's physical strength (Manus), Giles's knowledge (Sophus), Xander's heart (Animus) and Willow's magic power (Spiritus). This showed how much the Scooby Gang as a collective was important and how each contributed something of value.
My personal favourite moment of the Scooby gang is that moment in Season 7's Chosen before the final battle when they just discuss normal things - it just sums up the group so completely. They talk about going to the mall after saving the world, leading Giles to declare - "the earth is definitely doomed," which echoes the end of The Harvest in Season 1. Its as though they have come full circle.
🙂