Probably death via brain vivisection. Essentially, you're given a local anesthetic to allow you to feel no pain as the top of your skull is removed, beautiful music and images play in front of you, then, over the course of several hours, a well-trained neurosurgeon removes pieces of your brain in sequence, beginning with high-order functions relating to the senses. Slowly the music looses its rhythm, beauty, and melody as you become tone-deaf, and then develop amusia, you watch as the beautiful images loose their color and vividness.
The surgeon continues to cut, removing more and more of your brain, but making sure to leave as much of your situational memories (Long term and short term) and higher reasoning skills intact. As you reach the end, you've lost your ability to speak and understand language, your ability to hear, see, smell, or feel anything, you've forgotten how to walk, to tie your shoes, tto use a fork and knife, or how to smile, you can no longer even wriggle in your restraints. All that's left is the parts of your mind that let you remember what you've lost, the reptilian, primitive core of your being, and the frontal sections that let you realize all the implications of what's happened to you, before finally the knife comes down, beginning to peel back layers of your reasoning one by one.
You feel yourself becoming less and less intelligent, your thoughts simplifying, loosing coherence. Eventually, you are reduced to nothing but a few neurons generating primal fear with memories of being something greater, before the surgeon makes the final cut, severing the medulla oblongata. Your heart and lungs stop instantly, and your remaining brain cells die over the course of several minutes, starved of oxygen.
