In the original D&D set, every monster's AC is (barring special circumstances) one of 8 literal Armor Classes. Extending the chart to account for penalties to hit, I think the best possible "effective" AC is -3 (Plate Armor, Shield +3, Ring of Protection, long-range missile from shooter with poor dexterity). Considering the armor and shield alone, that would give -1 -- which is hittable on a roll of 20 by even the weakest monster or character.
Supplement I introduces monsters with ACs beyond the literal range (notably the will-o-wisp with -8!). It also adds multiple factors that, by stacking, could theoretically give a character an effective AC as good as -22 ... (AC 2, -4 for dexterity, -5 for magic armor, -5 for magic shield, -2 for ring of protection, -7 for long range fire from weak bow, -1 for shooter's poor dexterity) ... or maybe even better (if I've forgotten something).
Cover or other factors could add penalties to hit. Note that rings of protection don't stack with armor for AC purposes in AD&D. Keeping that anyhow; setting aside the shooting factors (leaving AC -14); and treating old AC 9 as WotC AC 10, that would give a new AC of 33. (Call it 34 if you start from AD&D AC 10).
That would probably be considered ludicrous in most old D&D campaigns. It's not exactly un-hittable (21+ for a level 16+ fighter in OD&D, and the +1 could come from strength or magic; 18+ for level 17+ in 1E AD&D, or 20+ for as low as level 11, or optionally 10, by the standard table with repeating 20s). However, AD&D gave Indra only -12, and even if that is not the best among the gods it is pretty well up there. Even Odin (a common yardstick for "godlike" power) gets only -6.