Mine was just accomplished a few minutes ago (and what inspired me to make this topic). I was going through Swindler's Den with Wuuthrad, cleaning away the enemies as I came across a bandit hitting me with bolts of lightning. I charged up to her, smacked her with Wuuthrad to stumble her, and quickly performed a power attack. I went into the little cinematic kill, and as the bandit was crouching down in pain, her head was close enough to a nearby rock that my character used it as an executioner's chopping block and sliced her head clean off.
Even though I've had a few decapitation kills that were kind of cool (you can only decapitate with the appropriate perk in either the one handed or two handed skill trees), I just thought that how my character used the rock to aid him was just plain beautiful.