This is hard. Which one? Do you want to lose your ability to see wonderful sights, the face of your child, hear heavenly music, feel the touch of your loved ones, smell fresh baked brownies conjuring warm memories of your childhood or taste the symphony of ingredients bursting on your palate?
All is important. I can't pick one. But if I have to - Sight. Sight gives us the ability to process the outside world and how we need to act and react quickly. May be the most important to keep us safe and alive more than the others.