Cutting one of the femoral arteries would be your best bet.
"The femorals are the main arteries and veins to and from the legs. As you might expect, they carry a lot of blood, and an untreated cut to either one is quickly fatal. In one case, a thirty-four-year-old man bled to death from a 2 millimeter (less than Vs inch) nick to the femoral vein—and the vein is the low-pressure half of the circuit. They run next to each other, with the artery closer to the front of the leg, and are nearest to the skin at the groin, which makes a tourniquet hard to apply if that's the site of the cut.
While knives can be and often are used on impulse, one case showed the benefits, however temporary, of planning and knowledge. A physician applied a local anesthetic to his leg, and then neatly cut his right femoral artery lengthwise. I estimate that he would have lost a fatal amount of blood in about five minutes. He died quickly, and, probably, painlessly." - Geo Stone, Suicide and Attempted Suicide