I think the
Rules Cyclopedia version of D&D is the
best one ever produced. The core rules are simple and fun enough, but there are plenty of optional rules (e.g. skills, unlimited demi-human progression, weapon specialization, "death's door" rules, etc.) that you can adopt as you see fit. It is far friendlier to tinkering than 3.x, and much more streamlined and coherent than 1st or 2nd ed. AD&D. (The Basic/Expert rules, though also good, lack all the additional options that RC includes.)
Although not an OOP version of D&D (and not even a fully "in print" one yet!), you might want to check out TLG's "Castles and Crusades", which is heavily inspired by pre-3.x D&D (while using the d20 mechanic). A lot of people who play OOP versios of D&D were involved in its developement (not me!), so it has a lot in common with pre-3.x versions of D&D, while being in print.
Finally, if you are interested in pre-3.x versions of the game, I would strongly urge you to check out
Dragonsfoot.net, as it is a site devoted to OOP D&D (they also have forums for C&C, Hackaster, and Savage Worlds). Gary Gygax, Frank Mentzer, Steve Marsh, and a few other important people from the "early days" of D&D often post there.
Good luck!