A couple of days ago I was linked to Searchers of the Unknown, which is a one-page trimming-down of classic D&D. Despite being dubious about its limitations and holding on to descending AC at first I soon found both of these were good things.
Firstly the limitations meant you had very little to consider, as a player, mechanically. We encountered a pit very early on and rather than rolling athletics to leap over it we went back and unscrewed a door, using it as a makeshift bridge. This is extremely basic D&D stuff, but it felt so much more encouraged with such a minimal ruleset.
Secondly, descending Armour Class actually made sense for the first time. It was a roll-under target for attacks and was used for things like sneaking in a way that made the smaller=better idea actually fit.
Not to be outdone, I remembered that I had my own very-light system, The Adventurer's Tale. Although this game is very simple in itself the document is still pretty large with the optional rules, monsters and spells. Could I trim the core of it down to one sheet? I could. All the rules for creating an adventurer and playing the game are on the front of the sheet, with the back filled with spells and monsters for characters to encounter.
So here's the Adventurer's Tale One-Sheet Quickstart. Grab a few friends and run a pickup game today!