This writeup is a bit gross. Sorry.
There are only three places above the shoulders on the human body that are 100% fatal when stabbed with a knife. They are: 1) just underneath the chin when an upward stabbing motion is used and 2/3) between the ventral and dorsal collar bones on the right and left side, when a downward stabbing motion is used.
How do I know this? My fencing instructor taught me. (How does he know this? I didn't ask.*)
He told me that these three points are so fatal because the bone structure there is such that the blade will always be guided to a particularly nasty spot. In the case of the chin, the upper and lower jawbones will guide the tip of the blade right into the brain. In the case of the collar bone, the blade will actually be guided into the path of the major artery that supplies blood to the arm (this is called axillary artery, or so Lockheart tells me); when this artery is severed at this particular location, it will bleed internally -- thus no amount of external pressure can slow the bleeding, and no pressure point can be accessed upstream from the wound. The only way to survive this injury is immediate vascular surgery, since an uncontrolled arterial hemorrhage is usually fatal within minutes.
Just a note on targets that don't make this list: The eyes would seem obvious, but the eye sockets are very thick and bony, making it difficult for a knife to penetrate into the brain. The throat might seem the most obvious choice, but the cartilage surrounding the windpipe is surprisingly strong and springy, making it unlikely that an inexpert attacker would be able to penetrate the windpipe on the first try.
*And you don't ask. A man who spends his life learning about long stabby things and how they are used tends to know all sorts of grim and strange facts. Trust me.