Hm, for me? It probably has to be when I got the first 1000/1000 for Gears of War 2.
Why? Well because I had so many people telling me that I would never do it, that I would give up before I reached 50,000 kills and 1000 rounds. Well when I first got "Party it's like 1999" some people believed I will, but I still had a lot of my friends putting me down. Well when I finally did it, I got to have a fun day.