Majora's Mask, definitely. The soundtrack was wonderful (including my favorite Zelda boss music), the premise and primary mechanic entertaining and often genuinely creepy, the subquests involving and fun, and the dungeons challenging.
Following that is probably a tie between Wind Waker and Twilight Princess, the former for its art style and seriously fun exploration element, the latter for its story and fun-as-all-get-out dungeons (including some great bosses and weapons...Spider-Link, Spider-Link...).
And Ocarina of Time deserves a mention, naturally. Even though it's third in my own list, I still love the game to death. I still get chills whenever I hear <url=http://www.youtube.com/watch?v=O-xKPWePgRw>the final fight theme.
All in all, a series as old as me...and I love it.