Zelda's most viable KO moves entirely depend on where you and your target are on the stage. There are many factors beyond that, such as:
*Is the move at full power? If not, then you might want to think twice before failing to KO with it and still weakening it further.
*Will they be able to recover from this? (if they can't, then there's no reason to be picky about outright KOs)
*Are they good at Smash DI?
*Can you hit with the move to begin with?
If you really want one answer though, then it's Uair. This almost always sends people skyward to their deaths with only 90 damage or less. It's a disjointed hitbox with good range, but the hit window is very brief, and the move itself is slow, so people will dodge it like herpes. You'll have to trick people into it. After Uair, sweetspotted kicks are the next best thing, although those are difficult to land, too.
Since the "best" KO move you can use will always be context sensitive, you'd do best to look at how
all of your attacks send even with DI. There are even times when Ftilt and Utilt are your best bet at KOing your opponent, due to their angles and reliable, single-hit nature.
And since PwnyRide popped in, I feel that it's appropriate to mention that Zelda's "****" are not KO moves, and I'm still laughing about it.
