The scariest: Road of Death
That road that Daniel Craig's James Bond speeds through at the beginning of "Quantum of Solace" (along Lake Garda in Italy)? Scary.
But for pure fingernail-wasting terror, there's Yungas Road in Bolivia. Called the Road of Death, Yungas has long stretches of thousand-foot drops and no guardrails. It's winding and tight, and it rains a lot. People die there. It's no place for backseat drivers.