wow, I've been sitting here for awhile debating over this, it's hard to come up with a total overall best moment...
There were several moments in MGS4 that were crayZ awesome or made me angry or sad or whatever can a whole game be a moment?
I'm also gonna go mega oldschool here and go with my completion of earthbound, I had never played the game before, but it has totally burned itself into my heart, now I know why everyone who has played it raves about it so much, it was hilarious and heartwarming and Im so happy I took the time to play it!
I know my best multiplayer moment would probably be from SSBB though, in an epic duel against my arch-nemesis(roommate) samus vs falcon, which has been our mega deathmatch since SSB(64). we had 8 or so people hanging around playing on acouple screens, and everyone was watching the match, after like a minute of an edgegaurding battle on our last stock we both knocked each other in opposite directions and i ran onto the stage and somehow managed to get hit with knee, after which he followed me off the stage and followed through with a totally unnecessary FALCON PUNCH!!!!!....because I just wasn't dead enough. yeah, I lost, but it was so hilarious, and everyone was laughing so hard. And to this day, whenever my friends and I mention SSB, it pretty much goes to "hey, remember that time you got falcon punched while you were like 10 miles away from the edge of the stage?".
That was kinda long, but, I think that you gotta include a silly multiplayer moment that lives in infamy sometimes. Moments playing games with your friends can be easily as memorable as the most heartwrenching, or incredible scene from your favorite RPG or FPS campaign playing by yourself.