Depends.
If using bare hands I'm quite partial to picking them up and ripping them in half. God of War/Prototype style (ripping vertically though, not horizontally like Kratos). It's simple, brutal and sends the message loud and clear: You're next.
Another option would be to tackle them down and start ripping them to shreads with my hands/teeth, inspired in L4D's Hunter or those two scenes in Neon Genesis Evangelion (if you've seen it, you know what I mean). There's something cathartic about the complete berserker mode.
If using a sword or scythe, then I like the idea of turning it upside down, sticking it in their stomach, then turning around and using my own shoulder as a lever to cut them in two, with possibility of upwards projection and gory shower.