Awesome question.
I'm undecided. Not being able to hear or see are the worst disabilities to have out of the five listed imho, so I definitely wouldn't choose either one of them. I'd probably go with something I could cope with the most.
Not being able to speak would suck, but if I was mute at this very point in time then I'd probably carry around a little board that I can write on and show everybody so they know what I'm thinking. I'm not sure how often I smell anything, so I'm not sure if that'd be a huge loss. Of course it would suck, but maybe not as much as being mute. As for no taste, that wouldn't be a big deal if I ate a food I dislike, but it would hugely suck not to be able to enjoy anything else.
I'd probably go with no smell. It's probably the one I'd cope with the most.