Well not counting the Master Sword (I absolutely love swords), I'm going to say the Clawshot.
Pretty much the same reason as everyone else, it would be cool to grab things from a ways away, and I could see it being quite useful a lot of the time.
The Double Clawshot. I could get to several places much easier, and it would be quite fun travelling through that method too. I know it has been mentioned before, but I have to say that the Spiderman feel to this would be a great addition to making it a real life item.
Hmm, I suppose I'd like the Zora Tunic. You can breath under water so wou don't have to worry about going up for air while swimming. And perhaps you could find something at the bottom of a lake and now you can get it with the Zora Tunic!
If the Master Sword isn't an option, then I'd go with the double clawshots. There are so many times when I'd want to have them, usually when I'm stuck at a stop light. It would be so nice to just go right above everything with the double clawshots.