This day's question revolves around my favorite adventure and it should be one I've run, too. I'll keep it short today. As far as published Adventures go, I did buy a lot of those and downloaded some more (back when WotC had a lot of the adventures online for free) and I don't need to mention the huge amounts of free goodness the OSR is producing on a regular basis. I've ran only a hand full of those, all in all. I'd love to run more and maybe will do so in the future, but for now my choices are pretty limited.
On the other hand it wasn't that difficult to find a favorite adventure (the favorite dungeon location is part of tomorrow's question). It's the HackMaster variant of the infamous Temple of Elemental Evil:
The Temple of Existential Evil (T1-4)
|I'd have gone with|
The Temple of Mental Evil (with the handy acronym ToME)
We didn't finish the whole module, but we had a great showdown in the moat-house. Played for 3 years (real-time) in the area, too. As far as I'm concerned, it was the perfect mix of HackMaster charm and the classic module. Would have loved to run the whole thing, but the group fell apart. Someday, maybe.
HackMaster 4E really was the first retro-clone. Now that I think about it, they were quite successful at the time, "old school" was one of their most favorite words and they offered pdf support (maps, battle sheets) for their modules and they revived all the classic old modules for their game. I'm glad nobody paid attention at the time...
Sometimes I miss running HackMaster.