1E and house rules.
A LOT of house rules, even after multiple serious attempts to reduce the amount.
1E AD&D is great... for what it is. You don't jump in a Model A and head onto the freeway thinking it'll drive the same as a BMW 535i, a Prius, or Challenger. If you're taking 1E D&D onto the modern RPG freeway expecting it to be competitive you need a lot of modifications. Doesn't mean you don't do it, just that it comes with expectations of NEEDING house rules and still playing quite differently than newer editions. Similarly, it's RIDICULOUS to me to draw comparisons and say things like 5E can replicate the "feel" of 1E. Bollocks. Part of the FEEL of 1E is its very clunkyness requiring individual creativity in rules from the person running it. Even a souped up Model A isn't the same thing as a 1965 Mustang. But that works the other way too - a 2020 Porche won't EVER look, sound, feel, and handle the same as anything you do to a Model A.