First D&D product you ever bought. Do you still have it?
After wracking my brain all day, I've come to the tentative conclusion that the first product I personally bought, as opposed to receiving as a gift from someone else, was the adventure module DA3: Temple of the Frog. My understanding is that this was written for the original Blackmoor campaign, but was adapted for use with the Known World of the Expert Set by making Blackmoor a part of the Known World's distant past and invoking time travel to bring the PCs into the setting.
One of my brothers was to step into the DM's shoes to run this one, with me as a player, so I didn't read it until much later, though the cover illustration, with characters seemingly firing some sort of laser guns against giant frogs clambering out of a swamp, made it sorely tempting. Despite my brother's inexperience at DMing, I remember it being an entertaining adventure, with lots of investigation, lots of people we weren't sure whether or not to trust, and lots of sneaking through the temple and trying not to arouse too much suspicion amongst the priests and lay members of the frog cult.
Of course, I eventually did read through the entire thing, and realized that there was a lot of content that my brother probably just didn't know how to utilize. I still do have the module, but I'm not sure if I'll ever run it. It might make a cool one-shot adventure some day.