Hmmm…most old-school? Is this a question of actual vintage, or of playstyle? In terms of their actual vintage, I still have my Moldvay Red Box D&D Basic Set book (though not the box, and only 1 of the dice), which was the first RPG thing I ever bought. I have since acquired a number of Dragon magazines that predate that by several years, and are probably my oldest RPG anything. Though in terms of really epitomizing what I think of as “Old School” RPing, it’s probably a multi-way tie between Grimtooth’s Traps (the original), with it’s adversarial-GM attitude and completely ridiculous traps; DragonRaid, the Christian proselytizing tool that takes the worst adversarial & railroading aspects of early RPGs, and weds them to more railroading and a dubious and self-contradictory approach to witnessing and morality; Spellbounds, which reads like someone’s home rules, vintage 1980; Actor’s Book of Characters, a supplement for World Action and Adventure, which the author managed to justify as earning a dozen college credits and being his thesis; and Fantasy Wargaming: the Highest Level of All which is this interesting mishmash of semi-authentic realistic setting elements focusing on things like the nameday of your character, and bog-standard fantasy rules for the era, all with a tone of revelation.
Perhaps the one game I own that feels even more old-school to me than Grimtooth’s Traps (largely because none of the games I ever ran or played or even heard about actually did the adversarial GM thing, much less the all-but-cheating nature of those specific traps) is the original Arms Law, back before Rolemaster was even an idea. Well, I don’t actually have that, I have a photocopy of the crit tables from the very first edition of Arms Law, which a friend had bought. Rather than dropping in the whole Arms Law system in place of the combat rules in D&D, as it was intended to be used, my friends and I had decided to keep the simplicity of D&D and just graft this one piece on because the crit results were so much fun. I know that a crit was only a possibility if you rolled a natural 20, but I don’t remember whether we used a 2nd attack roll or a 2nd unmodified roll or the degree of success of the original roll to determine the degree of the critical. I might be able to puzzle it out if I were to look at those tables, but I haven’t busted them out in decades.