Tied in 1st place are Ocarina of Time and A Link to the Past. Why are they so good? Because they're as fun today as they were back when I played them in 1998 and 1992 respectively (Yes I was 2 years old when I first played Alttp).
Ocarina of Time has awesome temples (Fire Temple), creepy temples (Forest Temple) and hard/annoying temples (Water Temple) which makes a perfect mix. The items are fun to use, like the Hookshot and the Hover Boots. The people you meet in the games are memorable. It's just fun to mess around with your ocarina. The action is phenomenal and if you've played this game as a kid, you really felt like you were Link and you had no choice but to embark in the adventure to go save the world from destruction. Being that it was the first 3D Zelda game, it sure left an impression and they really nailed it.
Why is A Link to the Past so good? Because it's a simple game. It's easy to pick it up and play without having to break your brain to figure out how it works. It's fun hunting for treasures and heart containers. The temple bosses are well thought and some are really hard if you're bad at video games, like the Skull Woods boss and the Thieves' Town boss.
For me, the modern Zelda games (from Twilight Princess onwards) are still very good, but not as memorable as the ones I've play as a child. I guess being an adult makes it harder to enjoy games as much. I don't know.