Which OSR adventures are most useful?

The things that have been most useful to me, running D&D:

Dyson's Delve I & II - for my swords & sorcery Wilderlands campaign. Tons of great dungeons, and then tons of beautiful unkeyed maps to make your own.

Most of the adventures from Basicfantasy.org - Adventure Anthology 1, The Glain Campaign, and Monkey Isle in particular are all great. Saga of the Giants looks cool too. They are in the style of Moldvay-Cook/Marsh Basic/Classic D&D adventures, the printed versions are sold at-cost from amazon. Most very good, and unbelievably good value. I use them for a Mentzer Classic D&D campaign set in Karameikos.

I also enjoyed Venger Satanis' Liberation of the Demon Slayer (for the Wilderlands game), tons of inventiveness but it's a bit of a mess, the maps need re-keying.

I think the key for me is that OSR adventures need to be modules I can plug in to my own campaign. I recently bought Stonehell & Dwimmermount - these are well done megadungeons, Dwimmermount especially is fantastically well presented, but they are designed as tentpoles for your campaign, not plug-in elements.

If you wanted to kick off a megadungeon-focused OSR campaign in OD&D style, then I'd recommend Dwimmermount. If you are doing your own wilderness sandbox dotted with dungeons in the Judges Guild mode then the Dyson's Delves (you can plug them in to eg Rob Conley's Blackmarsh or Points of Light). If you are doing episodic 'dungeon of the week' - the default Moldvay & Mentzer Classic D&D approach - then the BasicFantasy.org stuff is best, though much can also be used for sandboxing.