The first time they ever came out with an update. I think medic was the first one. I remember when everyone was scrambling and to do all the achievements to get their new gun. It was exciting and fun because you got awarded with a cool new weapon. It really set the stage for future updates and these updates are probably why I'm still playing this game...
I don't remember what it was called or anything, but when they introduced the huntsman. At that update, they introduced a lot of cool new weapons, that there were many bugs that allowed me to get very many amazing kills.