tl;dr A giant-sized core ruleset that nonetheless offers the best take on an Advanced version of D&D you are likely to find. I never really looked at the Adventurer Conqueror King System, though I had heard good things about it, as an elaboration of classic D&D with a proper domain game. When the new Imperial Imprint (II, aka v2) came out, I decided to spin up my drivethrurpg account and plop for the pdf. Let's start by saying this is not classic D&D. It is evidently based  on BX/BE D&D, with its math and level progression advancing only to 14th level.  But the scope and detail of this game is far more ambitious than anything Classic and those who enjoy the restrained core of Classic D&D will find the wealth of options and systems overwhelming.  It is rather an alternate attempt at making an Advanced D&D, based on Classic D&D.  And it is epic/monstrous in scope. Let's look at word counts of core rulebooks (not including monster manuals): ...

Somewhere in the Dragon Magazine issues I've been perusing lately, Moldvay comments that the part of the Basic set he is most proud of is B60 "Dungeon Mastering as a Fine Art", where he gets to impart his own experiences as a DM to newbie DMs. I leave the entire passage transcribed here below without further comment: The success of an adventure depends on the DM and his or her  creation, the dungeon. The DM should have the dungeon carefully  mapped out before play begins. Even so, a DM will quickly find  that it is impossible to predict every possibility. After all, there are  several players, and only one DM! It is not unusual for players to  find a solution, or pose a new problem, that the DM has not even  thought of. It is very important for the DM to be flexible. It is important that the DM be  fair , judging everything without  favoring one side or another. The DM is there to see that the ad venture is interesting and that everyone enjoy...

Smaller numbers are better. It's not always  true in RPGs, but it is generally true. It's easier to work with in terms of calculation, but perhaps more importantly it makes it easier to intuitively gauge the significance of the numbers. You can feel the impact of a +1 on 1d6 more than you can on a d20. The older I get, the more I appreciate smaller numbers. The art quality in OD&D may be lacking, but the art direction  was pretty dang good. Number creep started with Greyhawk, continued in AD&D, really took off with 3e, before being scaled back a bit in 5e, with its notion of "bounded accuracy". 5e's notion of bounded accuracy still yielded a greater inflation of numbers than Classic D&D (Holmes, B/X, BECMI, Cyclopedia) which held back a bit on that front but still had slightly higher numbers than pre-Greyhawk D&D. One of the most obvious parts where number inflation took off is hit points. And in this regard, I quite appreciate the HD scale in OD...