The first module I ran was B2: Keep on the Borderlands.
Because it came packed in the Basic set that I got for Christmas.
The first module I bought would be X1: The Isle of Dread.
Because it came packed in the Expert set.
(*) Completely off topic, but the effect of having played AD&D and then looking at 5e was a bit amusing. I read through the books and thought, "Well, this has certainly been modernized and polished, but for the amount of time that has gone by, not much has changed." I did not learn the The Rest of the Story until I started reading this forum.![]()