I bet you're going to get a *lot* of opinions on this. Here's mine:
I think "Classic" D&D would probably be a good choice. The version commonly called B/X or Moldvay/Cook is my favorite version of Classic D&D:
It is out-of-print. While used copies are plentiful, if that is a problem, the next best thing would be
Labyrinth Lord. Labyrinth Lord has a lot going for it. It's a one-book solution, it's available in print (including hardcover) and also in the form of a free PDF, and it's well written and has fantastic and inspiring interior art. (That last factor is one advantage it has over the
Rules Cyclopedia, another "one-book" Classic D&D volume.)
You mentioned THAC0 as a factor against 1e. THAC0, itself, was little-used in 1e; most often 1e relied on the combat tables, although THAC0 could still be used in many situations, if you wanted to. If your distaste for THAC0 includes a distaste for "low numbers are good" armor class, in general, the the above games might not be the best choice. However, there are two alternatives I recommend:
Basic Fantasy RPG shares much in common with "classic" D&D, but it uses "higher is better" AC numbers.
Swords & Wizardry shares much in common with 0e 1974 rules, and has a "flip-AC" system where numbers are included for both AC styles: you pick the approach you prefer. Lastly, while OD&D(1974) is my personal favorite, I don't recommend it for your particular situation; I think you would find the original rules difficult to learn from. However, if you lean in the OD&D(1974) direction, Swords & Wizardry would be an excellent and accessible alternative.
I also recommend that you take a look at
RFisher's Classic D&D site, and perhaps my own
musings on OD&D.