I can't smell, and I have a moderate to severe hearing loss, so this was already a hard enough decision. I think if I had perfect command of all my senses but had to give up one, I'd go with hearing. To be sure, it's a wonderful and rather useful sense, but I wouldn't be who I am now if I could hear like everyone else.
I voted taste. As far as I'm concerned, taste just gets in the way. I think the world would be better off if people couldn't taste. Imagine the reduction in obesity, as eating ceased to be an enjoyable activity, ending all gluttony.
Yea, some people eat out of habit, etc etc but I'm talking big picture.
Sight- You'd run into things, put on the wrong socks, mistake everyone for everyone else...
Taste- Wouldn't be able to taste your grandma's cookin
Hearing- No music
Touch- You could get cut/stabbed/shot and not even know it until you were bleeding to death o.O