If I lost sight, I'd die by walking in front of a bus.
If I lost taste, it would just suck having to eat texture.
If I lost touch, how do I feel pain?
If I lost smell, the only thing I'd have to worry about is poison gases and such.
If I lost hearing, I'd die by walking in front of a bus.
The thing about losing smell is if there was a gas leak or fire you wouldn't even know til it was too late. Plus all the gnarly farts you'll be missing out on.
The safest one overall I think would be taste. Although it would suck not being able to taste delicious foods on the plus side you'll be able to eat 100% healthy since taste don't matter anymore.
Smell, no contest.
I love eating and drinking, read lots of books and comics, carry my iPod with me wherever I go, and love touching stuff (not what I meant). While I do like smelling the aroma of freshly-baked cookies, being unable to smell the diner's toilet or the farts of others is fine by me. ;)