I am not sure it is fair to have only one proudest moment. After all, some moments that I was intensely proud of in the past would be effortless for me to replicate nowadays. Still, a recent moment that made me feel real good was when my best friend and I won our first 2V2 match of StarCraft II.
We went up against the same guys twice in a row. I played as Protoss, my mate played as Terran. Our enemies played as Protoss. In both games, they Photon Cannon rushed us, something we had no real idea how to encounter. They won the first game. In the second game we fought off their initial rushes. I rushed out Photon Cannons faster to defend my base, and my friend simply relocated. We settled into a long siege, with our opponents throwing down Cannons over the map by the dozen, and eventually taking out all my mining capabilities. My friend supported both of our armies with his economy. We eventually took out their initial bases with a combination of Immortals and Marines. But they relocated, and walled themselves off with yet more Cannons. After a fifty two minute siege, we managed to break through their defences and finished off their final base.
This would now doubt have been an effortless match for more experienced players. But for my mate and I, it was an exhilarating triumph against stacked odds.