As for me... The rarest killing moves I've seen are the ones that are only do-able when your target is on their knees (Which happens when they have like 1-5% of their health left). So far, only saw this once and I didn't even did it! One of my Followers was fighting this bandit guy and then the bandit went to his knees saying the normal surrender stuff.. Then it happened! My follower put his right foot on the bandit, pushing him down onto the ground, then taking his sword and impaled it thru the guy's body then pulling out once the guy was dead! I was like "OMG!" and I've been playing since 11/11/11 and I jsut saw this in April!
Now.. For my faves that I love to see... I love climbing onto dragons and stabbing their faces or smashing my axe/hammer into their skull.
You?