1. Wario's half charged fart especially if it's done as an edgeguard and results in a stage spike. Fact: this is made 1000x better if you get Wario's "LMFAO" victory screen afterwards. For added hilarity, try doing it on Green Greens. Best KO I ever got: I was playing a King Dedede, used the waft, he bounced off one of the "brick walls", richocheted off a brick coming down from the top of the screen, and ended up dying in the bottom left hand corner of the screen. As you may have expected, he quit the very next match.
2. Mario's fsmash. Honestly, it's just poetry in motion as you see the remnants of the flames as it goes into slow-mo just before the victory screen as your opponent flies off screen.
3. Ness's Psycho Crusher (aka PK Thunder 2). This is made even more satisying if you did it through an elaborate mind game or they were trying to spike you offstage to end the match.
4. Any overpowered attack after a broken shield. Seriously, nothing's better especially if the match was really long or if you were playing a really annoying character like Diddy. Ike, Snake, or King Dedede's fully charged fsmash are the first that come to mind but who can object to taunting and then resorting to a Falcon/Warlock/Donkey Punch to the face?
5. Ness's Bthrow. One of the few throws that can still reasonably kill. The experience is made even better if you wait just a little bit before doing it in order to watch your opponent sweat and really soak in the onslaught of doom.
6. Snake's C4 especially if the match ends with you offstage with a higher percentage and your opponent is about to attempt to edgeguard you. "Now!" "Wait... what?!?" "BOOM!" "CRAAAAAAAAAAP!" *twink* GAME!
7. Any move that results in you dying shortly afterward or at the same time like King Dedede's inhale or Bowser's Flying Slam. What's even better than this is an all or nothing fast falled spike like using Ganondorf's Wizard Foot (down B) or Diddy's dair.