I'm running Adventurer Conqueror King System again, and it's like I've come up for air without knowing that I was underwater.
One of the most refreshing things about ACKS (and B/X D&D upon which it's based) is that this game knows what it is. And, importantly, I know what it is and I like what it is.
There being no such thing as D&D means that, over the years D&D has accumulated a lot of cruft designed to make the game accommodate more and more playstyles. As the editions have progressed, the "centre of gravity" of the rules has moved with these rule additions, making the game as written appear very different than what I know as D&D. I can still play 3.5e as the D&D I know (and know how to run), but someone coming to it fresh is going to see a game of high magic, Wealth By Level, optimisation, splatbooks-are-required, and magic item shops. 3.5e doesn't know what it is, and making it run like the D&D you know is a trick that requires patience, hard work, getting everyone on the same page, and vicious elimination of any implications (rules- or fluff-based) that don't fit your gaming style.
ACKS is great because it's pretty much just Basic D&D, and Basic was early enough in the game's history to be very focused on doing one small set of playstyles well and smoothly. ACKS adds some refinements to the rules, but they're well-considered to mesh with the existing game's playstyle – they don't change its essential nature or move its centre of gravity. ACKS is D&D as I know it and love it, with a few rough edges filed off, and it runs like oiled silk on the game-execution engine that is my brain.
So that's something to consider when playing a game: Does it know what it is? Is it honest with itself? Is it full of rules that are there as compromises between a diverse and divided player base, or divided game history? Does the game communicate well what its aim is? And finally, is that something you like? Is it something your players like?
These criteria are why I don't run 3.5e and probably never will again: it has too many moving parts that are central to its design that only exist to support design goals that are contrary to what I want out of a game called "D&D".
It's why I don't like 4e: it does what it does very well, and it communicates what it does well, but it's not honest with itself in that it pretends to be "D&D" for everyone. (It's a pretty minor lie by omission, but it was still a lie in the service of brand promotion. I'm pretty sure Hasbro wouldn't have let WotC say "this is a particular kind of D&D, and it might not be to your taste, so you might want to stick with 3.5e or try another RPG entirely".)
It's why I have trouble playing even 2e again: it's all very much about the player's individual character story and their special-snowflakeness (which is great on its own), but the combat system and class/level design are relentlessly hostile to that, which makes the system incoherent. And, it's why 1e doesn't really work well for me: it's too baroque for a simple dungeon/hex-crawl, and I don't like the epic questing that it's design for.
ACKS is great. It's in my personal Goldilocks zone.