Farewell Hyrule King
Even if you do NOT like The Wind Waker (I'm not a big fan of it myself) you simply must admit that the song Farewell Hyrule King (search it on your WW soundtrack or on YouTube if you do not know what I am refering to) justifies the existance of that game entirely. That song is reason enough alone to play it through. It's a masterful variation of the already grand Hyrule Castle theme Koji Kondo made for A Link to the Past.
(Although, one could also easily say that the Wind Waker manga justifies the game's existance totally, too. ;) Brilliantly simple and hilarious.)