This is a topic worth considering, although I do believe that it's too early to definitively consider many classics for the 4E era.
What does "classic" mean for adventures, anyway? The conventional definition is that a "classic" is exemplary of its style or that it has a timeless quality. I'd posit that for D&D, a classic adventure is one that is so well-known and well-received that it has become part of the lexicon we use to describe the game. An adventure that almost everyone who has played the game has played, or has at least heard discussed as a model of design. In this sense, I'd consider the following adventures (across several editions and publishers) to be classics:
The Keep on the Borderlands (B2): Sure, it's only a few caverns chock-full of monsters near a place to rest. But the sheer number of players who have been through the Caves of Chaos certainly qualifies the adventure as part of many player's first experiences with the game.
Ravenloft (I6): Ravenloft has been held up for decades as a paragon of good design. Not only did Ravenloft draw upon a literary and cinematic basis, it also used familiar D&D monsters in ways that fit the story well and the adventure has some serious replay value, even if you've played through it before (through the use of the villain's variable goals). I've run Ravenloft in every edition of D&D so far and players have always loved it.
The Ruins of Undermountain: This one is a classic simply because it embodies the old-school love of vast complexes that can serve as the basis of an entire campaign. I understand that it's currently being used in the RPGA as the basis for material for Living Forgotten Realms.
The Sunless Citadel: Many players first experienced 3E through this adventure. It's sort of like the Keep on the Borderlands for 3E in that respect. Not only does it showcase the best of a new edition, but it incorporates some interesting new ideas as well. The story showcases a new type of monster, and it's notable for introducing a druid as a villain and includes one of the most fan-loved NPCs ever (Meepo).
Red Hand of Doom: For a while, WotC didn't make any printed adventures. They got back into the swing of adventure design with Red Hand of Doom, which not only had an interesting story, monsters, and NPCS, but also incorporated material from its (then) current history of published products. Seeing the victory point system from Heroes of Battle in actual play was a real treat. It also had interesting twists on classic monsters, such as a druidic lich with ghostly animals. More than anything previously published, this adventure showcased the flexibility of D&D's mechanics and source material.
Keep on the Shadowfell: Although this adventure gets mixed reviews, I still consider it a classic. Many 4E players discuss their experiences in this adventure when they join a new group, which I think makes it worthy of inclusion. It's a favorite with DMs too, since it includes the neat maps for several encounters (albeit most of them are reprints).
Although I think their status as classics is disputable, I'd like to give honorable mention to the following:
Dragonlance: The original Dragonlance saga is fondly remembered by many older gamers. Ten years ago, I would have included the original Dragonlance adventures as classics without hesitation. However, it's getting tougher and tougher to find people who actually played through these adventures, despite updated support in the 3.5 era. Players just don't seem to much care for Dragonlance any more.
Rappan Athuk Reloaded: Rappan Athuk scratches a primordial gaming itch within me like no other product. It's so reminiscent of my early days of gaming that I'd be thrilled to run this entire thing from beginning to end. Sure, it's just a big dungeon crawl--but it's a good dungeon crawl. If you enjoy that style, Rappan Athuk can satisfy in droves. However, I'd hesitate to call it a classic simply because so many people don't enjoy that play style.
What does "classic" mean for adventures, anyway? The conventional definition is that a "classic" is exemplary of its style or that it has a timeless quality. I'd posit that for D&D, a classic adventure is one that is so well-known and well-received that it has become part of the lexicon we use to describe the game. An adventure that almost everyone who has played the game has played, or has at least heard discussed as a model of design. In this sense, I'd consider the following adventures (across several editions and publishers) to be classics:
The Keep on the Borderlands (B2): Sure, it's only a few caverns chock-full of monsters near a place to rest. But the sheer number of players who have been through the Caves of Chaos certainly qualifies the adventure as part of many player's first experiences with the game.
Ravenloft (I6): Ravenloft has been held up for decades as a paragon of good design. Not only did Ravenloft draw upon a literary and cinematic basis, it also used familiar D&D monsters in ways that fit the story well and the adventure has some serious replay value, even if you've played through it before (through the use of the villain's variable goals). I've run Ravenloft in every edition of D&D so far and players have always loved it.
The Ruins of Undermountain: This one is a classic simply because it embodies the old-school love of vast complexes that can serve as the basis of an entire campaign. I understand that it's currently being used in the RPGA as the basis for material for Living Forgotten Realms.
The Sunless Citadel: Many players first experienced 3E through this adventure. It's sort of like the Keep on the Borderlands for 3E in that respect. Not only does it showcase the best of a new edition, but it incorporates some interesting new ideas as well. The story showcases a new type of monster, and it's notable for introducing a druid as a villain and includes one of the most fan-loved NPCs ever (Meepo).
Red Hand of Doom: For a while, WotC didn't make any printed adventures. They got back into the swing of adventure design with Red Hand of Doom, which not only had an interesting story, monsters, and NPCS, but also incorporated material from its (then) current history of published products. Seeing the victory point system from Heroes of Battle in actual play was a real treat. It also had interesting twists on classic monsters, such as a druidic lich with ghostly animals. More than anything previously published, this adventure showcased the flexibility of D&D's mechanics and source material.
Keep on the Shadowfell: Although this adventure gets mixed reviews, I still consider it a classic. Many 4E players discuss their experiences in this adventure when they join a new group, which I think makes it worthy of inclusion. It's a favorite with DMs too, since it includes the neat maps for several encounters (albeit most of them are reprints).
Although I think their status as classics is disputable, I'd like to give honorable mention to the following:
Dragonlance: The original Dragonlance saga is fondly remembered by many older gamers. Ten years ago, I would have included the original Dragonlance adventures as classics without hesitation. However, it's getting tougher and tougher to find people who actually played through these adventures, despite updated support in the 3.5 era. Players just don't seem to much care for Dragonlance any more.
Rappan Athuk Reloaded: Rappan Athuk scratches a primordial gaming itch within me like no other product. It's so reminiscent of my early days of gaming that I'd be thrilled to run this entire thing from beginning to end. Sure, it's just a big dungeon crawl--but it's a good dungeon crawl. If you enjoy that style, Rappan Athuk can satisfy in droves. However, I'd hesitate to call it a classic simply because so many people don't enjoy that play style.