I have to say...
Wario Waft. That would be THE WORST way to get knocked out in real life. But yeah, seeing Wario's body glow orange and seeing your opponent at about 60% or higher, and finding a good chance to just.... Let it all out. Instant satisfaction.
Jigglypuff's Rest goes without saying, but dang! Your opponent is at mid to high percent, you find yourself practically inside of them for a second and TING! You asleep, they dead. RIP.
Mario's/Donkey Kong's forward air. Or anyone with a spiking move for that matter! Just seeing them go down, down, down... And down.
Villager's Tree of Destruction. It looks so hard to land, but you'd be surprised at how many people run into it. Plus you sometimes get a cute little remnant of it to throw to its death.
And my last mention is dedicated to my sister, who loves to use Luigi - Up+B. She loves getting hits with it, especially when it kills two with one hit like it did one time.