This game was my introduction to the OSR, but it's much more than that.
Adventurer Conqueror King not only gives you an updated and clever reconfiguring of the original B/X Dungeons & Dragons rules, it delivers on the promise of high level play.
So many D&D games make reference to "gaining a stronghold and followers" at level 9, but they offer no interpretation of what that means, what that would be like. It's often relegated to story dressing, and doesn't get the attention that, say, combat does. ACKs fills in this sorely needed gap in a way that is largely compatible with most fantasy medieval games.
They say that what you give experience for in an RPG determines the kind of play you encourage. In this way, ACKs encourages ruling kingdoms, engaging in magical research, founding religions, running a thieves guild, or embarking on mercantile ventures as well as it does things like killing foes and stealing their treasure. Not only this, but it encourages doing these things boldly: timid rulers satisfied with their holds won't gain XP, but ambitious conquerors who expand their realm will.
Of course, these rules alone are worth the price of entry, and are easily ported to your game of choice, as the economic ideas underpinning the XP rewards are consistent enough to survive translation as long as you properly anchor to common worths. But the actual rules for playing adventurers are good too. You get a servicable expansion of available classes that simultaneously preserves the feel of "race as class" while giving people playing demihumans more options than usual.
And as if that wasn't enough: this system is well supported. If you find that you like this, you will be amazed at the supplements available that can further expand your games into anything you've ever wanted to run.
[5 of 5 Stars!]