Ocarina of Time. I'd hate to get redundant with my explanation of why but here goes nothing...
Ocarina of Time may have had a rather basic story, however, it had some of the best graphics for its time and new features such as lock-on spread like wildfire throughout the industry. The game added a whole new degree, a layering on top of the already intricate overworlds and dungeons the franchise was known for. If that's all it did, I wouldn't be here. It's a masterpiece to play and that's why Ocarina of Time is my choice.
Wind Waker. It never fails to bring a smile to my face, sailing will always be a wonderful way to pass the time, it has every element of decent story telling within its narrative such as deep characters, melancholy, sacrifice, an epic backstory to explain its world, mystery and wonder. WW was more than a quest, it was an adventure, a tale with true mileage and lasting resonance. something that no other Zelda really has in my eyes. It may not be the best Zelda game but for me it is the deepest and most life affirming Zelda tale.
Used to, it was Majora's Mask. Now it's Skyward Sword without a doubt. I've probably put over 300 hours into that game. No joke. I just don't get bored with it. I'm kinda shocked at it myself. (MM is still a close second, though.)
Such a hard decision! Personally, it was a tie between Skyward Sword and Wind Waker. WW was the very first Zelda game I ever played, it is so much fun, not too easy, not too hard. I have a lot of emotional moments tied to it. SS is just SO MUCH FUN! I love the motion controls, all the dungeons, bosses, music, etc. But the thing that made me choose WW was the emotional connection I have to it, I would never want to loose that.
This is an impossible choice! I'm tempted to say Adventure of Link, simply because it's the most difficult and it would keep me busy the longest, but I think I'd have more fun with a game like Skyward Sword or Majora's Mask. Maybe even The Wind Waker.
...Can we just not discuss what a horrific future that would be if I only had one of these fantastic games to play?
Ocarina of Time. It has a good amount of dungeons and fun bosses, not to mention sidequests and trading sequences. The music is absoulutely beautiful, each a catchy tune I find myself humming every now and then. There is even a master quest version for the Gamecube and 3DS, with new puzzles and the overworld in reverse. Now that's a game I could play for the rest of my life!