ooh, I used to think about this in class when i was bored all the time! i'd decide who in the class i would kill first if i could and there wasn't the whole 'guilt' thing attached to it. (I'M NOT CRAZY, I SWEAR.)
I'd probably kill them with a two handed sword. I haven't practiced my fencing in a while.. It would be fun. I could give them a sword, teach them the basics, fence them, and then slit their throat when I got bored.
Either that or I would set them on fire.
EDIT: actually, I think the funnest way possible to kill someone is by making them SUICIDE. I'd give them a gun and have a bullet proof barrier between us, and I would basically convince them that they want to die. I'm a cold hearted person, and have incredibly miserable views on life (I just don't really care), so I could manage this.