The game of my youth: AD&D 2nd Edition. By the time 3rd edition came out, I had a measure of system mastery, I could run it in my sleep, and I felt comfortable creating rules content for it that wasn’t unbalanced. I played 2nd edition for its entire life cycle, though I didn’t truly appreciate some of the settings (like Planescape and Spelljammer) until their time was long gone.
Of course, no game is truly dead as long as you still have the rules and can still play it. I play multiple, so-called “dead” games each year at Gary Con, but with the advent of 5th edition D&D, I doubt I’ll go back to 2nd edition AD&D for any reason. 5th edition captures the feel of AD&D for me, and if I was to run it at a convention, I would stick with 1st edition AD&D (though the two games are similar enough mechanically that new players wouldn’t even notice) because it’s what everyone thinks of when they think of the classic version (BECMI, B/X, or Rules Compendium D&D, aka “Basic” is fantastic, too; I would run that again before I run AD&D).