My favourites:
1. The Sunless Citadel. It's more or less been elevated to the role of a classic already. It's a solid low-level dungeon crawl with a few memorable encounters, a real challenge that forces you to think creatively to survive, and the fan favourite kobold Meepo.
2. "Mad God's Key" from Dungeon #114. Best chase scene I've seen in a D&D adventure, along with a balanced adventure design and a number of cool fight scenes.
3. Red Hand of Doom. It's long and epic, utilises standard-issue evil humanoids to good effect, and will teach your players that occasionally, retreat really is the best option.