this is a really really tough one, but i think i would have to go with taste. it is the only one that does not seem absolutely essential, and it would make it easier to eat healthier, but there are so many good foods out there that arent necessarily unhealthy. its a great pleasure to give up, but it is the only sense i can live without, if only just. (im not actually obese, i just find food one of the cheaper (mostly) yet better things to enjoy in this world. just in moderation, because excess of anything is kind of a bad thing.)
EDIT: oh. didn't know we had more than five. in that case, i would go without temperature. heat bugs the crap out of me, and extreme cold does a bit as well. while it can be refreshing, it is hardly essential, and the pain from extreme heat/cold would still register, so not necessarily an essential survival thing.
you know? on second thought, or third rather, ima stick with taste. temperature plays a hardly noticeable but really influential role.
you know what? fuck it, i just can't decide. its impossible to imagine life without any single one of my senses since i find use for them so often. i really do feel sorry for anyone who doesn't have a certain sense. i just cant imagine what that would be like.