Title says it all ! It can be anything, from the first time you had discovered your favorite super bounce, or the time you had 8 of your friends playing BTB destroying to get a high rank in it.
Mine would probably be beating my first pro team online in H3. It was Triggers Down when they had just came off of an event win. Or getting my first 25 by myself in Team Snipers on Halo 2.
edit: actually it might have been when I found the flying dumpster glitch in H3 with my friend (Subonik) It was so ossim that we had Aft3rShocK capture and post it to H3F :B (the thread got dug on dig.com and the record for the most users ever online was broke with that !) Sadly the Mythica link was taken down because.....Mythica. I also had heard from someone that the glitch caught enough attention to be featured on G4. I think I heard AoTS at the time. Regardless if true or not, still cool =]
here's the youtube search results for all of the videos I had inspired LOL i just found this
Halo 3 flying dumpster.........not to be confused with Halo: Reach dumpster video below... for the lulz