Obvious choice here is penis. Sooner or later as you age you most likely get erectile dysfunction etc. so you will effectively be neutered anyways.
After a certain point, when you have enough sex you realize just how overrated an experience it really is unless you are super rich / famous and can get a threesome every night with different women or something crazy like that. Even that would get boring after awhile though.
Sight / hearing are so essential even for basic functioning that the thought of losing them is absolutely terrifying to pretty much everyone.
Between sight and hearing though as referenced in the other post, I would say lose the hearing. Yes it would suck, but can still enjoy a lot of things with sight alone.
The truly expendable sense though has got to be smell and / or taste. Losing sense of taste would actually be a good thing in a way; no incentive to eat sweet fatty foods.
