I would definately choose the Master Sword. It is Link's weapon of choice in most games. Plus, the Master Sword is legendary. It is "The Blade of Evil's Bane". Hopefully we will find out why in Skyward Sword.
Even though I must admit the Master Sword is a classic, the Four Sword is obviously FAR more useful. I mean really, a sword that can split you into four versions of yourself? Count me in!
The Master Sword is a classic, but I'd have to go with the Goron Sword. Even though Link needs two hands to handle it, it's well worth the work to get the sword. Plus, you help out that big lug of a Goron on top of Death Mountain!