I'd say it would be the two extremities when it comes to their awareness of their impending doom. This means that either they knew it was coming and felt completely powerless to stop it, or when they suddenly die in a very shocking way.
From surprising them with a shotgun at point-blank, to seeing their doomed faces as you drop a rather large explosive/vehicle/object on their head.
Personally, I do love the 'hidden explosives' trick. You hide a C4 charge or something on the opposite side of a doorframe, and detonate it once they've walked right past.