I've got two:
First one is also from 2010, back from when I was a freshman. The feeling that you get from walking through the tunnel in Atlanta for the first time and into the Georgia Dome to see all those FIRST fans cheering for their teams like there's no tomorrow is something to behold. It really gets you going.
Second one is the entire Razorback Regional for 2013. While we didn't quite take the regional with us, boy was it a lot of fun. Seeing all the robots that came from KC improved, including realizing a new, much more effective strategy for our robot, and getting to meet some absolutely amazing people down there too. (And possibly because so many people down there were like "Oh, you're team 2410? Do you know CalTran?" "Yeah, I know him..."
) Topping off the regional would be getting invited to the second alliance, captained by our favorite KC team, The Kuh-nig-its (1939), as well as the "host" team, Baxter Bomb Squad (16), was the absolute best way to have ended my FRC career as a student.