From Necromancer Games There are a good bunch that should become classics. Topping my list is The Grey Citadel, Lost City of Barakus, Rappan Athuk, Tomb of Abysthor, Doom of Listonshire, Vault of Larin Karr, Hall of the Rainbow Mage, and others. However, the popularity of Necromancer has never seemed to be great, so i suspect they will be popular in the same manner as old Judges Guild stuff.
Dungeon has definitely had a bunch of good ones over the years.
I like Goodman DCC's, but none of the ones I have used, which is only about half a dozen or so, have really "come alive" for me. They have been fun, challenging, and entertaining, but just haven't had that "special" element to them. Now I own about 24 of the DCC's and some of them read to be really cool, but I haven't taken my campaigns in directions that would allow them to be used.
The setting, DCC 35 is pretty frickin good, though. Plus I am looking forward to Whiterock.
I also like a lot of the Kalamar mods, such as the Coin Trilogy, and a few others.
Paradigm put out a few that may become classics.
Green Ronin definitely did with some of the Freeport modules.
I think Giants Skull and NeMoran's Vault by Fiery Dragon have a shot as well.
Anyways, thats all my tired brain can think of at 3 AM.