L1
Sekiro
:p

UltraKill's parry punch. (I play Steam Deck so it's X).
Also Izuna Drop from Ninja Gaiden Black (XYXXXY)

Biased K9999 main reporting to say pretzel inputs in KoF/SNK fighters.

Dizzy's instakill in Guilty Gear XX Accent Core +R, completely impractical and borderline impossible to pull off in an actual match but it's still funny lol
[2]8462P+H
or ig↓(charge), ↑←→↓P+H

Komaki Tiger Drop: R1 + △
From the Yakuza series (choosing between one of them, it would be Yakuza 5)

for ultrakill, I think the projectile boost makes more sense (parrying your own shotgun projectile)
(With Shotgun) Shoot, Immediately Parry
a few more suggestions:
Celeste, wave dash: Jump, [Pause], ⇘Dash, [Pause], Jump
Sonic 3, spin dash: ⇓(hold), A, A, A, A, A, A, A
Starcraft, building a shit ton of dragoons: Click, D, Click, D, Click, D, Click, D, Click, D, Click, D

God of War PS2, infinite roll: Roll, □, Roll, □, ...

Thank you everyone. I'm glad none of you mentioned Super Smash Bros. Melee.

Potemkin Buster from Guilty Gear Strive (632146+P)

DMC from Ninja Theory. along with DMCV, it have the best gameplay combat in the saga

aw hell yeah, Raining Strike in Sifu is the GOAT

hell yeah it fuckin is dude

Haohmaru heavy slash (5C or 5AB depending on game) single handedly makes SamSho worth playing

soul steal in castlevania sotn (←→↘↓↙←→ + ◯ or ☐) screen nukes are sick and i like you how you move around https://youtu.be/cr989oGkDGw&t=57

Bubble Bobble
press the fire button on the joystick

Super Mario Odyssey Cappy Jump:
B, X, LZ, Hold Y