Phenomenal. I say this because of the entire thing.
It's a simple story, a story of a hero saving a princess, but must defeat somebody to do so. But it's more complex than that at the same time. There are other people to help, things to acquire, and things to learn.
Not to mention, there is a lot to ask in the background. Think about it, there is still questions about OoT after about 12 years. It's just phenomenal.
Unique. The Zelda series are in some peoples eyes your standard fairytale save-a-princess-from-a-great-evil-being adventure, but it isn't. The series suprise you all the time and I think that there aren't any games like this in existence. they are a real masterpiece. One thing I always liked about the series is that you can upgrade your life, weapons,etc. without the use of an Exp. system (except in Zelda II then) but as reward for finding secrets or defeating bosses and other challenges.
Nonserious answer: superultramegacalifragilisticwonderfulwhatisthisidonteven because this word describes every aspect of Nichijou- I mean, Legend of Zelda
Serious answer: There are many different aspects and many different words than could describe this amazing series, but I think that the word breathtaking describes the series as a whole because it always takes my breath away (cheesy, I know) because they always surprise me with new features in the game, but they still try to keep the same, classic elements in each game.
Emotional - In a good way! The series is able to get you so into the game that you are able to experience certain emotions while doing various things, interacting with various characters or traveling to certain places. For a video game, that is a rare and spectacular feat. For an entire series, it's phenomenal!