I don't think it's about scenes of redemption so much as moments of choice that crack his façade. The big one for me is in 'Return of the Jedi,' when Luke's being tortured and Vader just keeps watching, his helmet shifting between Luke and Palpatine. You see him weighing a lifetime of monstrous decisions against this one screaming point of pain. It's quiet, but the tension is in his stillness. He's been a weapon for so long, and in that moment, the weapon hesitates.
Then the act itself, throwing the Emperor down the shaft, feels almost frantic. It's not a grand, heroic gesture with a speech; it's a desperate, physical lunge to stop the pain. What sells the redemption for me, weirdly, is the aftermath. Him asking Luke to remove the mask. That's the real moment—not killing the Emperor, but choosing to die as Anakin, to look at his son with his own eyes. The machine is gone, and for a second, the man is back. It's heartbreaking because it's so brief.
All the 'Clone Wars' stuff adds layers, but that final sequence in the throne room is the core of it. He doesn't get to atone, he just gets to choose one last time, for someone else.