I love or like all of the Zelda games except Zelda 2, so it's hard to choose. But Majora's Mask was such a refreshing change of pace for the series that I have to pick it. (I haven't finished Breath of the Wild, though.)
I thought I hated the concept of the repeated three days until I tried it and saw how brilliantly they executed the idea. The game was much darker and more bizarre than anything else in Zelda which I appreciated, and it was interesting how it took what you learned about the world in OoT and used that to subvert your expectations and create a nightmarish "mirror universe." All this from a game I thought was going to be a cheap reuse of assets.
In fact the only thing about MM I don't like is that it marked the first absence of a Kondo original soundtrack in a mainline Zelda game. MM's soundtrack is great, nevertheless.
This is a really difficult choice, but I went with OoT. Maybe I'm just nostalgic, but its place in the series was an important precedent for how they handled basically all of the following entries up until BotW (and even then, it's still somewhat formulaic once you get past the initial 400+ hours of awe and wonder.)
Then again, I really don't think I've ever truly disliked any of the games. OoT and MM hold a lot of nostalgia and emotion for me, and I thought TP was excellent as well. The whole series is very good at subtle references and call backs to the other games, which is a pretty good way to be, I think.
In regards to game play, though, I gotta give it to BotW. Link Between Worlds was also really fun
Zelda 2 gets a bad rap. It's the most skill-based game in the series, and the additional mechanics make it the best 8-bit ARPG, bar none. It's also got a surprisingly coherent world, with things like unlockable shortcuts that make geographic sense, and NPCs that were (for the time) useful and interesting.
Objectively, there are superior games later on, but Adventure of Link has been near and dear to me my entire life.
Believe it or not I picked Oracle of Seasons. Both games from the Oracle series were fantastic. But I found Oracle of Seasons to be more fun because it blended more simple puzzle solving with hack and slash. Basically what I loved about the Game Boy Zelda games.