Zardnaar
Legend
Also, the intro modules in the rulebooks need to be part of the conversation. Lots of folks still curse Bargle's name, meaning he had to traumatize a lot of players. There's also the tower of Zenopus. And the AD&D DMG dungeon has been reprinted in multiple later books.
They can't compete with Phandelver or Keep, but I think a lot more people have played through those than we realize.
Also, the most popular Dragon and Dungeon magazine adventures were also very popular. I ran Citadel by the Sea -- a great classic PCs versus orcs adventure -- many times, for instance, and Fedifensor, by virtue of being one of the first planar adventures and featuring the githyanki when they were new and very hot, was also super-popular.
Dungeons great but they didn't sell that many copies relative to the big B/X, 1E and 5E modules.
Solid contender for 3rd place would also be Tyranny of Dragons/CoS along with one other 2E modules or X1.
After that it's probably various 5E modules. Only around 3 or 4 B/X or 1E modules broke 200k+ sales.
B2 a long time with AD&D crossover as well.
Nothing certain but B2 and LMoP are the biggest selling adventures of all time.
LMoP is the second biggest selling adventure of all time just using that partial glimpse they gave us 5-6 years ago NA sales only. Probably the biggest selling ever. Can't 100% say that and there's various estimates of B2 as well.