I'd like to see peoples epic finishing moves! What was the best way you (or someone in your game) finished off the enemy?

The best I've seen was a artificer-paladin in my game who attempted to sacrifice himself to save the town. With the giant cheese monster climbing the cliff, he performed a thunderous smite whilst leaping off the cliff, plummeting into the beast. The player rolled a nat 20, and I proceeded to add the damage incurred from the fall (using custom falling rules of my own devising which are somewhat more punishing), and applied some flexibility as the cheese monster was soft.

The paladin/artificer, Thunderbrand, managed to deal 189 damage in this one move. The monster had 188hp.

I then asked them for a dex save, and they rolled high. They landed in the fondue pot below, which broke their fall and left them with just 3hp left!

It was a bit of a DM Fiat (strictly speaking they would have taken a similar amount of damage themselves) but I hadn't expected them to net the creatures hitpoints in a single attack!

What have been your best finishing moves?

I was playing an Aarakocra druid and I stabbed a skeleton, flew up with the skeleton and threw it to the ground where it shattered into a million pieces. Not really a grand climatic thing but it was cool.

There is a portal shining out of the ground pure flaming daylight in the middle of the courtyard, lighting up everything in a 20 ft. radius. Everything beyond that is pitch black with the smoke that can't be found anywhere but the realm of death.

Two of us had already dropped, one was turned to stone, and the other two just one hit from going down. The Barbarian and Warlock are standing on opposite sides of the well, just inside the light. The two remaining gorgons can just reach the Barbarian and attack without pause. The first misses. The second...

does not. The Barbarian goes down hard. The Warlock standing just on the other side of the well looks around at his fallen comrades and sees the positioning of the gorgons. The Warlock closes just enough distance and uses bonus action Relentless Hex, teleporting to the opposite side of the gorgons, right next to them and launches 2 rays of Repelling Eldritch Blast.

Both fall just onto the edge of the portal. Suddenly they begin roaring in pain as the portal pulls them in and leaves behind nothing but dust and silence.

(edit: spacing)

My players were inside a castle’s treasure vault, and the rogue had found the ancient king’s crown hidden in the walls, he failed a wisdom save and the spirit residing in the crown took control of him. The others tried to take it off but the benefits the rogue got from the magic item were too strong. The paladin, with about 5 hp left, miraculously succeeded in a series of rolls and stuffed his onyx dog into the mouth of the rogue and yelled the command word. Of course, the rogue was ripped apart but the spirit in the crown then passed to the dog, opening up a bunch of side quest opportunities. DM’s gotta DM.

A 1st level Human Bard and a 2nd level NPC Human Cleric were up against 8 bandits.  The Cleric used Burning Hands and wiped out half of the bandits.  The two bandits outside the fan charged and were killed, one by each character.  There were three remaining bandits after this point, all with 8HP of damage.  The Bard, seeing that they were unlikely to beat the three remaining bandits, tried another tactic.  He began forcefully walking toward the three bandits in an attempt to intimidate them in their wounded state, and the two bandits began to step away leaving only the bandit leader.

The Bandit Leader, having been the victim of a Vicious Mockery, in an enraged state lunged and swung wildly at the Bard.  The Bard deftly avoided the attack and lunged forward (rolling a Nat 20) driving his rapier [Descriptive Text Follows] deep into the bandit's left shoulder.  The bandit abruptly pulled his right hand to the wound in response to the pain bringing his weapon up and away from his body exposing his ribs on his right side.  The Bard, expecting this reaction to the pain stepped away as the bandit's blade swung up, and then stepped in to bury the blade of his dagger in his left hand between the bandit chief's ribs on his right side, burying the dagger to the cross piece.  There was a fountain of blood that came out of his side as he collapsed melting to the ground and wrenching the dagger from the Bard's hand, the bandit gazing up into the air with his eyes wide open focused on nothing.  The other bandits took to their heels not wanting to stay in the fight any longer.

We were finishing up an Ebberon campaign and our DM told us we would level up at the end of the fight. I got the killing blow so I asked him if I could use my Pact of the Blade feature for the first time and she said yes.

My Dhampir cut her own palm and used that hand to clamp around the enemy's throat, asking if he had any last words he wanted to say. When he started to, my character summoned her pact weapon for the first time in the form of a rapier crafted from her own blood. It went straight out the back of his neck, killing him.

As a DM: My first arc BBEG was a deathlock mastermind who was being forced to commit crimes against a town he loved when he was alive. He secretly hoped the party would stop him before it was too late - and the bard figured this out during the boss battle. So he walked up to the deathlock, cast Dimension Door, and dropped the BBEG from 500 feet in the air. 

I was playing a githzerai monk, and I smashed an ice mephit into "19,823" pieces with a broom handle.

Edit: I forgot to mention that I took 1d8 slashing damage from its death throes and died. Very anticlimatic.

Not a particular specific instance but my when my Cleric kills things it's often with blunt force of some variety, most often with either her mace or spiritual weapon and I often describe such as crushing skulls, caving in ribcages, and generally just smashing the crap out of things. The DM is fond of calling for dex saves when I do so because of how such methods of dispatching foes can be understandably messy. No damage involved, but with a dex of 8 and no proficiency in dex saves, Serena often ends up having to ask the party wizard to hit her with a half dozen or so castings of prestidigitation to clean off the blood, gibs, gore, and other unpleasant fluids and bits that she tends to end up getting splattered with by the end of most fights.

yesterday, my warlock finished off the BBEG we had been chasing for a solid half a year (or more, almost 3/4 of a year) it was a Ilithilitch (if i spelled that right) and it was a difficult fight. my warlock got some insane damage rolls on their four blasts and with repelling blast they got yeeted into the wall. I just imagined my character chain-gunning four blasts into the BBEG's chest and them being bludgeoned to death by being repeatedly slammed into a wall. wasn't the most epic way to kill someone, but it was awesome becuase of  the build-up!

In tonight's game as the DM, I rolled a crazy number of 20s.  There was an encounter with 20 bandits against the party of 6 level 3 PCs and 3 level 6 ranger escorts.  Toward the end the bandits realized this wasn't going to go their way and some started peeling out.  One ran away and a ranger shot him down rolling a nat 20 using his longbow.  I described it as the arrow penetrating his neck just below the skull and the bandit just face-planting into the dry leaves on the forest floor.

Right at the start of a Lost Mines of Phaldelver campaign I was playing, we found a camp of goblins on a tower. My bronze Dragonborn blasted his breath weapon at the goblins, and the whole tower burst up in flames and fell down. His breath weapon had already melted most of them but the fall finished the rest off. My dm described it as ‘You blast a torrent of lightning at the top of the tower, and 3 of the goblins start to melt, screaming in agony. The tower bursts into flames and starts to collapse, and the remaining goblins fall to their deaths.’

My party was fighting some drakes and my eldritch knight was almost dead and had just missed 4 attacks in a row they then finally stabbed a drake and Booming Blade exploded it. My party's eladrin rogue then tried misty stepping down the other drakes throat and that went... as expected.