The most dangerous that I can think of is Marth's Critical Hit. It still has a bad hitbox, but it's essentially a one hit kill across several damage ratios. Ignoring K.O potential, my favorites are:
1. Triforce Slash (Link)
It looks awesome, and it's the embodiment of awesomeness. I've nothing else to say, really, except that it could use a LOT MORE knockback and overall damage.
2. Great Aether (Ike)
Again it looks really awesome, a ton of hits strung into a beautified combo. What I like most about this is its KO potential - my bro killed me at just 76% damage [I air dodged for momentum cancel and still died -.- ]
3. Cook Kirby (Kirby)
This might not be terribly useful and has very low KO potential, but it's still cool looking (everytime I activate it, I say "salt, salt, pepper here" lol)
4. Light Arrow (Zelda)
I can't remember which one is the vertical and which one is the horizontal...but I like Zelda's more than Sheik's regardless. Shehot
5. Mario Finale (Mario)
I only like this one because it looks cool; it has a horribly hitbox.
6. PK Starstorm (Lucas)
Yay
7-9. Landmaster (Falco, Fox, Wolf)
Poysonally, I prefer the *** <3
10. Negative Zone (Luigi)
If I like anything, it's a ton of status effects. NZ gives JUST THAT.