I think I like the Four Sword the most, mostly because of it's ability to split the user into four, which is awesome. I wouldn't mind four of me running around, but the other staff might
Its appearance is pretty cool too, but I guess it's the cartoony styling that makes that happen. The Master Sword probably would steal my vote if it was in a cartoony style ... well I guess it was in the Wind Waker, but that's not how I would've imagined a cartoony Master Sword.
Even though the Master Sword is the Sacred Blade n' all that stuff, I doubt it could split me into Four any day of the week, although, keeping me safe in the Twilight Realm is a bonus, but only when infused with light.
Oh, another reason, it's the only sword in the Party Zelda Games, which are always amazingly fun with a crowd.