Probably Fox's shine. Better than Falco's shine because it only sets up combos on certain characters(and occasionally kills off the top/sets up for shine to up-B).
Only other moves I'd say are close are Jiggs' Bair and Rest.
Rest can be comboed into and can kill/deal damage, but it can also be punished hard with a stock trade or a powerful punish, and whiffing makes it risky.
Bair is great because it has great range, starts up and cools down sorta quickly, is used to wall, can shield pressure, combo, and kill.
Shine can shield pressure, but also cancel into grabs, upsmashes, jumps(and by that, aerials), set up for tech chases/followups at any percent due to its set knockback. Shine also comes out super quickly and cancels very quickly(and is invincible when it comes out). It also allows for infinites, which can obviously rack up damage and set up for kills simultaneously, which is certainly something rest/bair can't do. Oh yeah and the obvious shine spike part lol. Gimps ****.
Shine has damage racker/safer if whiffed on its side vs rest.
Shine has faster startup and cooldown/better damage racking/shield pressure/infinites/invincibility on active frames over Puff's Bair.