I'd probably choose to lose my hearing. Learning sign-language would be a difficulty I guess, but it'd show how many of my friends who'd actually bother to learn it to speak to me in person (or coming up with a clever work-around, like writing down what they're saying on SMS (not sending it, but showing me what they've written)). Also, hearing is the one thing I could loose without risking to lose my future job once I'm done studying.
And to everyone who voted 'smell', enjoy not tasting anything as well (ok, that's a hyperbole, but still)... An astoundingly large amount of taste actually comes from smell. Just a FYI.