Great Fairy's Sword. Was epic in appearance, dark but beautiful and ornate. It was also massive, possibly bigger than the Biggoron Sword (another favorite), and the two-handed heavy weapons are just epic. I would love Zelda to have more weapon variety in general, but the one-handed and two-handed sword distinction is pretty good in itself. I also enjoyed the Four Sword, and I just generally like the weapons that function in a unique way rather than just being slashing weapons. I'd love future Zelda games to use these special weapons more, possibly even having several in the same game so you have to choose which to use.
Another sword I like, although this one isn't usable, is the Twilight Sword that Zant used. It was awesome-looking, just to start. It also served as the actual weak-point for the Stallord fight's second phase, also cool. Finally, it had the
ability to revive the dead. We don't know anything about it except this, and it's a bad-*** fact.
It would be interesting to see this concept explored or even USED in a Zelda game. Plunging your sword into things revives or animates them? :3