A knife may or may not be the #1 most important survival tool based on your location and circumstances, but I think it is certainly the #1 most versatile survival tool. A good knife can add significantly to your ability to make a fire and a shelter, among other important survival tasks.

While it changes based on where I go and what terrain and circumstances I expect to face, my Most Important Gear List always has knife, fire, flashlight, whistle, and signal mirror on it. Fire making is where I have the most redundancy.