Over the years I've become something of a connoisseur of these kinds of rules, so it'd be hard to pick a favorite. If pressed, though, I'd probably go with the following little gem, which I would have actually used, had I noticed it back in the day:
HelmetsInterestingly, I have yet to find a comparable rule in OSRIC, but the Advanced Edition Companion does reproduce it.
It is assumed that an appropriate type of head armoring will be added to the suit of armor in order to allow uniform protection of the wearer. Wearing of a "great helm" adds appropriate weight and restricts the vision to the front 60° only, but it gives the head AC 1. If a helmet is not worn, 1 blow in 6 will strike at the AC 10 head, unless the opponent is intelligent, in which case 1 blow in 2 will be aimed at the AC 10 head (d6, 1-3 = head blow).
So what's your favorite obscure rule from AD&D? Bonus points if you actually use it in play.