I think that classic status in 1e had more to do with how many people played the module than how good it was. Although, I have to admit that modules like GDQ and B2 really do have a lot going for them, so long as the DM is good enough to fill in the gaps and make things come alive. Several years ago I wouldn't have said so, but having re-read them a year or two ago I will now. I never really cared for Tomb of Horrors, but I respect it's place in the classics catalog.
As someone else said earlier, some of the best modules for 1e were I6 Ravenloft and I3-5 Desert of Desolation (although Pharoah and Oasis of the White Palm were much better than the Lost Tomb of Martek). However, I don't think that these modules are as considered classics to the same extent as GDQ, B2, or A1-4 because they didn't come out when the game was young and get played by nearly everyone.
For the same reason, it will be difficult to judge what the classics are for 3e. Many of the third party modules are great, but it's hard to say how known/played they are in your average D&D circle (not composed of EN World/d20 junkies).
With that in mind, I think that the most likely candidates for classic status will be Sunless Citadel and Forge of Fury. IMO, both are great dungeon crawls in a traditional way and so have an element of nostalgia, but they also have a little more. Sunless Citadel's opposing groups and the interaction between them make it so even more so than most modules, two runs through it are unlikely to turn out the same. Forge of Fury is more straightforward but is well-put together and the tactics and motivations of the monsters make things memorable (especially the running opening battle with the orcs). I think that RttToEE might obtain a similar position to the original Temple of Elemental Evil - it seems to have great name recognition but I'm not really sure if I'd want to run the whole thing.
As for third part modules, as I said before, I'm not sure whether they will be able to obtain classic status with the edge that the official modules hold. But some of them are really good. Of Sound Mind is excellent all around. Crucible of Freya is excellent as a campaign pack with the free downloads but the main (printed) adventure is not as memorable by itself. The Freeport trilogy surely seems to have left a good impression on everyone - I haven't checked it out yet, I guess I better do so before it becomes any harder to find.
As for myself and my players, I certainly think that both Sunless Citadel and Of Sound Mind will be remembered fondly in future years. Both were really fun to play and I ended up being able to tie them togethor in some interesting and memorable ways.
