30 Greatest Marvel Movie Villains
3. The Winter Soldier - Sebastian Stan (Captain America 2 & 3)
The Winter Soldier is one of the MCU's great successes, beginning life as Captain America's pal Bucky Barnes, before being captured and experimented upon by HYDRA, transformed into a ruthless, programmable killing machine.
In his titular movie, The Winter Soldier is a force of nature, nearly killing Nick Fury and proving a rare adversary capable of standing toe-to-toe with Cap physically.
Morally, Buck is something of a mixed bag, because his real personality frequently bubbles to the surface and recognises his friendship with Cap, only to become supressed and/or confused again due to his programming.
This thread continued in Civil War, for though Bucky was by no means a straight-up villain, that didn't stop his programming causing him to lash out against the Avengers and create a ton of trouble thanks to Helmut Zemo.
As far as well-rounded villainy goes, The Winter Soldier has it all: intensity, presence and a compelling backstory.