I was worried it'd be a bit too common a choice, but seems not. I'd go with the Monado. First off it's material is awesome, it reminds me a little of Ryukendo, being halfway between metal and some kind of sleek plastic. A nice futuristic stylish feel. Second, it's shape is dependent on it's wielder. Even it's sword form in the game is cool, but it can be any sort of weapon so as to suit my personal taste/fighting style.
That alone would a solid candidate. But then add in the fact that it has several different powers and gives you the ability to see important events in the near future, and that about seals it. Also with it's limiter on it can't be used to hurt humans, so I could swing it around with total abandon as well.