Most memorable, seeing my first girlfriend die in front of me. Thats rather.... memorable...
Greatest? well, i've had a lot of "great" moments that I think are great. Less are considered great by my friends, and most where accidents or consequences.
people in my community think my greatest moment was I stopped a guy that robbed the gas station that I live near. The guy was armed, and he was running out. then i walked into the door (completely and utterly oblivious to everything like I usually am) and saw him running at me and just sorta braced so I wsnt knocked over. Well, he hit me (and to an extent through being used as support, the door), fell back, lost the gun, lost most of the money, and then three other customers tackled him, while I just stood there wondering what the hell happened.
... Wound up getting free gas for a year that year.
personally though, I think my greatest was I stopped a rape. Out park isnt exactly... safe, at night, and has/had a lot of criminal activity around it. I suffer from a minor case of insomnia or something like it, cause I never seem to sleep even if i feel tired. So I was walking up at the park, mostly cause I dont feel that threatened in it and I kept near the road. But as I was walking, I saw a guy that was triyng to rape a woman, and isntead of calling the police, or leaving the situation like a rational person, I ended up walking up to the guy, had a small scuffle, and sent him away with a few new bumps, bruises, and cuts to take home myself.
But, I wound up keeping her safe, called a friend to take us home, and the night just went into another day. Though I got a thank you card in the mail from her, so that was nice.