Heh, mine are (in no particular order):
1. Spotting Guitar Hero being released in JB when luckily on a day off from work with my mate. Just HAD to stop by my mate's brother's work on the way home to grab his brother's copy, then proceeded to do co-op for the next 3 hours straight (only breaking to grab drinks or to "use the facilities", and return to prise the guitar out of my brother's hands!!).
2. Experiencing Halo 3 in widescreen goodness, and finally "finishing the fight".
3. Loading up Live for the first time, some 6 months after getting my 360. It had its share of dramas (frantically trying to find a way to keep my achievements and progress after stupidly creating my account on here, rather than on the system. Worked out OK, only had to recomplete 3 games, so not so bad), but scrolling through all the demos and arcade games made me instantly regret not buying some points cards to start with.
I don't know why I put this here. I guess I just wanted to be unique... just like everyone else.