It's hard to argue with the lists of classic modules that have already been provided, but I'd like to offer up a few more details.
SPOILERS below. (Is it still a spoiler all these years later?!)
U1 - Sinister Secret of Saltmarsh
Ideally it would be great to have 3.5 updates (or whatever Dungeon mag. is planning) of the entire U-series, but if only one can be done, U1 it is.
U1 is a classic because it takes the stereotypical setups and goes beyond them. The setup of the adventure is that the PCs are investigating a haunted house. But as it turns out, the house isn't really haunted; it's being made to appear haunted in order to keep the townsfolk from becoming aware of the smuggling operation being based out of the house.
As the PCs investigate, they uncover move clues and peel away more layers of the onion that is the "Sinister Secret" of the title. It's not just smuggling going on - it's weapons running. And why are the lizardmen acquiring weapons? And meanwhile, what was the old alchemist doing in that house in the first place? And who is the mysterious Ned Shakeshaft found in the house?
And that's just the first module! With U2 and U3 the plot continues to unfold, and the PCs face growing threats which can be dealt with violently, diplomatically, or stealthily.
That's good adventure design. The framework of the adventure is set up and then the PCs are thrown into the mix. If the PCs do nothing, events will unfold without them. But the PCs are well positioned to alter events to their liking. And have a satisfying conclusion if they stick things out to the climax.
One weakness of the U3 is that the final battle takes place "off screen". But with the new miniatures skirmish rules, that battle could be played out - it could be tremendous fun for the PCs to control the allied forces of humans, sea elves, lizard men, and potentially locathah and merfolk against the sahaugin forces.
I'm also of the opinion that U3 in and of itself represents a fine tactical challenge for D&D players. The party must infiltrate a difficult environment, accomplish specific objectives, and above all avoid raising a general alarm which would bring down literally hundreds of hostile enemies upon them. Even if the players don't care about Saltmarsh and its inhabitants, they can approach the sahaugin fortress as a challenge in its own right.
I2 - Tomb of the Lizard King
Can you tell I have a thing for adventures featuring lizard men?
This is a classic adventure because of its heroic scope and the variety of locales and encounters the PCs come across. The setup does require a fair amount of railroading - more than would be considered acceptable by today's standards - but once the plot starts rolling, any adventurer worth the name should want to track down Sakatha and put an end to his reign of evil.
And that's another thing about I2 that I like: Sakatha has great villain potential. I feel that he's not as actively bad-ass as he could be - he just sits around waiting for the PCs to invade, even after
scrying repeatedly - but DM'd more intelligently he can present a formidable challenge to the party.
Let's see what else I2 has:
* Assassination attempt on the local lord? Check.
* Deranged, charmed guardsman? Check.
* Ambush at the old bridge? Check, and how! This is a terrific set-piece battle, even if the PCs know about it beforehand.
* DRAGON? Check.
* Hoards of undead? Check.
* Trapped rooms? Check.
* Evil wizards? Check.
My lord - what DOESN'T this adventure have in it?!
Weakness: again, the railroading; and once the PCs reach Sakatha's lair, it's a pretty linear dungeon romp. But dungeon romps are classic, and good ones are fun. This is a good one.
I look forward to finding out more about what Dungeon mag. has up its sleeve. Any revisitation of the consensus classic modules would make me happy.