Before we actually start defining third edition classics, we really ought to go with some standard for what a classic is. If it is only, "Everyone played it.", we might have a hard time making any of the 3 ed. modules classics because unlike the early 1 ed. modules everyone doesn't play them.
If we define classic as 'good' or 'timeless', we are going to have a hard time (IMO) picking good modules out of the 1st ed. 'classics'. In terms of design, most of them are terrible. 'Example of the sort of 'dungeon crawl' that typified early module design' might be the best we can say about them. GDQ especially. The A series isn't much better. Though we all played it and loved it when we were 12, B2 is mindless drivel and almost every 1st time DM has turned out homespun modules as good.
If I was going to pick 'classic' 1 ed. modules, my list would look something like this:
I6:Ravenloft - Hickman and Weiss took the dungeon crawl to the next level in this module. Not only did it have the best map of any dungeon ever printed, but it introduced the concept of the reoccuring over villain who acted to counter the PC's moves during the course of the adventure. And, of all the overvillains since, Strahd has to be one of the most memorable. Plus it featured one of the most atmospheric settings that a D&D module has ever taken place in. One of the earliest attempts at telling a cinematic story.
S1: Tomb of Horrors: Of all the 1st edition 'Dungeon Crawls', this is the best. It's traps and obstacles have never been surpassed and seldom equaled. A module designed such that thoughtful play trumps high stats. Even when revisted, in the quite good 'Return to the Tomb of Horrors', the sequel failed to capture the feel of the first because so many of the problems boiled down to fights with a monster.
I3-I5:The Desert of Desolation Series - Hickman and Hickman's work on TDoD series laid the foundation for essentially every module that followed that was written by anyone.
U1-U3: 'Danger At Dunwater' Series. Essentially the first attempt at a real story in module creation, with the first example I'm aware of of the 'plot twist' in a published module. Yes, the UK got there first.
UK1: 'Beyond the Crystal Cave' Speaking of firsts. How about the first module designed to be solved without combat? While everyone else was looking at the RPG as simply some sort of new twist on the wargame, this module was exploring the RPG as, well, ROLE playing.
The DL series: 1st editions crowning achievement. The epic story in interactive form. Sure, there was a bit of railroading, but the execution, the maps, the mythic-cinematic feel, and the incredible atmosphere made this the crown jewel of many a 1st edition player's experience.
Other nominations?
I haven't any real experience with 2 ed. and 3 ed. modules. Of the 2 ed. modules, the best read was 'The Axe of the Dwarven Lords' (Return to the Tomb of Horrors is a close second.) Skip created one of the coolest dungeons since Ravenloft, though I suspect if I'd played it (rather than just read it), that I'd have found it got tedious. Anyone play it?
I haven't played any 3 ed. modules, but I've yet to even read one that I made me think 'I want to run this'. (I guess I should pick up the Freeport series.) Monte's RttToEE might be the best done of the bunch I've seen (and if you ask me with the most 1st ed. feel), but I seriously doubt any module that is mostly dungeon crawl is going to keep my jaded attention for long. Still, I bet its a blast for a new player with the right DM. I didn't think Heart of Nightfang spire went anywhere that hadn't been gone before - and better. In alot of ways, it was I6 without the cool atmosphere and some ideas borrowed from RttToH.