Basic D&D. Mentzer's basic set, but Moldvay's expert set; for a brief period when I first got into it, you couldn't get the Mentzer expert set. Either it was still a few months from being released or the places I knew of that had D&D (a nearby toy store and, of all places, a fabric store near my school!) didn't carry it. My first two purchases from a proper gaming store - Hobbit's II in Edmonton - were the Companion set and the first edition of DC Heroes, on the same trip; my sister bought some Judge's Guild module, old and out of print even then, which I still have, along with Bunnies & Burrows!
Does anyone else with the Moldvay expert set remember being really disapointed when the Companion set finally came out - and thieves didn't get the ton of new abilities the Moldvay set promised? (Mimic Voices, Power of Distraction whatever that was, something about climbing upside-down, and a couple of others I can't remember off the top of my head). Even so the Companion set seemed like far and away the best D&D product I had seen to that point, and I still felt that way for some years. Still ranks pretty high, actually.