The 9th Level Enchantment spell Power Word Kill outright kills any creature with less than 100 hitpoints.
I've had a look, but I can't see anything that would be similar via any melee action of feat, creature-, or PC ability.
So my question is: Is there anything that allows a melee attack to strike down a foe to either 0 hit points (rolling death saving throws) or outright kill them using one attack?
Effects with saving throws would also answer the question.
There's a class of legendary magic weapons, Vorpal Swords:
When you attack a creature that has at least one head with this weapon and roll a 20 on the attack roll, you cut off one of the creature's heads. The creature dies if it can't survive without the lost head. A creature is immune to this effect if...
It has a bunch of limitations, but against most creatures it has 1 in 20 chance of instantly killing your opponent.
The Quivering Palm ability allows a monk to bring an opponent down to 0 hp directly:
When you use this action, the creature must make a Constitution saving throw. If it fails, it is reduced to 0 hit points.
Do note that this is a 2 step ability and only the first step requires being at melee range (an unarmed strike). The actual effect of bringing the target down to 0 hp only requires being on the same plane of existence. Still, this comes close to what you want.
This could be achieved by doing enough damage to trigger instant death:
Massive damage can kill you instantly. When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum.
As an example of how you could achieve this, the assassin rogue could pull this off with their death strike, assassin, and sneak attack abilities:
Sneak attack. Beginning at 1st level, you know how to strike subtly and exploit a foe’s distraction. Once per turn, you can deal an extra 1d6 damage to one creature you hit with an attack if you have advantage on the attack roll. The attack must use a finesse or a ranged weapon. [9d6 at 17th level]
Assassinate. Starting at 3rd level, you are at your deadliest when you get the drop on your enemies. You have advantage on attack rolls against any creature that hasn’t taken a turn in the combat yet. In addition, any hit you score against a creature that is surprised is a critical hit.
Death strike. Starting at 17th level, you become a master of instant death. When you attack and hit a creature that is surprised, it must make a Constitution saving throw (DC 8 + your Dexterity modifier + your proficiency bonus). On a failed save, double the damage of your attack against the creature.
These three abilities working in tandem can produce devastating results, up to two times 20d6+5 (average 150) damage in a single attack at 17th level, enough to trigger instant death on many targets.
Another way to achieve this is the Way of the Long Death Monk's 17th level ability,:
Touch of the Long Death. Starting at 17th level, your touch can channel the energy of death into a creature. As an action, you touch one creature within 5 feet of you, and you expend 1 to 10 ki points. The target must make a Constitution saving throw, and it takes 2d10 necrotic damage per ki point spent on a failed save, or half as much damage on a successful one.
Spending 10 ki points would deal 20d10 (average 110) damage on a failed save.
