I suppose it would have to be Moldvay Basic as I have run it several times over the last 5-6 years for friends and kids both.
Heck, Apprentice Blaster was running Apprentice Who through the Haunted Keep just a week or two back - summer vacation you know! It's a little weird to walk into a room and find my battlemat laid out on a table with a detailed map drawn on it and a bunch of mini's and character sheets scattered on and around it and realize that this happened without any involvement from me. It did make me smile though.
Honorable Mentions from the last few weeks:
- FASA Star Trek (80's) - research
- LUG Star Trek (90's) - planning, we might actually play this in the near future
- Deadlands (original flavor - 90's) - research as we are playing Reloaded now