Day 12 Old RPG that you still play/read.
For me this one is easy. Just recently at Storm-Con I ran a session of it and I had not run a game of of it in years. But to me the WEG D6 edition of Star Wars is like a close old friend. I recall more of the ins and outs of that setting then I do 1st or 2nd edition D&D.  It might be because I ran it for years and for a brief time I was running it daily. But I think I recall it so well and fondly because of the amazing adventures our party went on. All behind the scenes while Vader and Luke were crossing sabers in the for front our players were becoming their own legends.
At the convention this year I ran a Reunion game and for four hours not only did a few of my old gamer’s get back together but we rolled out and dusted off the old game system. And it still felt as fresh and fun 20 some years and two other system rewrites by other companies later. With all my crates of games boxed up and only a few precious systems still standing tall on my active bookshelf  It still sits.. more underplayed then played these days but I doubt I will ever have the hart to set that book in a box.
As a GM and a Player.. It has a place of honor few other games earn t he praise I have for D6 Star Wars.