Wow, this discussion is really morbid. I don't want to kill anybody but if I did, it would depend on what type of kill it was. If it was for hire, I'd just pop 'em with a gun, it's distant, it's less messy, etc. If it was personal, revenge, etc, knife. Get in close and just stab like Michael Myers, much more satisfying.
EDIT ~ Whatever the most painless way possible is. I don't know, lol. Drug overdose? I don't know if that's painless but I imagine I'd be out of it, lol.
Max Payne or Hitman?