two painful ones to list:
1. a good jab to the throat/windpipe area will make most people take a few moments to think about how much we take respiration for granted.
2. properly done, a sharp blow into the armpit will really smart. A knife or something truly sharp will sever a large artery.
there are lots of areas on the body where there is a lot of nerves but little skin/muscle/fat covering them. Like on the back of the hand, top of the foot.
a good cat fight move is the hairpull. of course this might be more for getting someone to comply with you rather than you helping them expire
..tm