Logically, I would think touch would be the obvious choice. Without out it, we would not be able to feel physical pain which and our bodies would not respond well to thinks such as heat, cold or even getting cut.
Pain is a response from our bodies to let us know that we need to back off, so with out touch, I would be bruised all over all the time from lack of touch.
Though ultimately I think I couldn't live without taste. God life would be so dull if couldn't enjoy food. Its an extremely unique experience and can only be matched by smell..and both of those go hand in hand.
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Though if i had to choose between sight and hearing, I couldn't live without hearing. I can easily fill in the pictures in my mind through imagination a lot better than imagining a sound. Imagination I believe goes further with sight than hearing...so I couldn't live without hearing.