I have a fairly decent list of OSR ruleset to the right of this post, but I'm sure it's incomplete.
Anyhow, my favorite OSR "Clone" (and a clone is in no way needs to be a prefect copy of the source material in this definition I'm using - it's more flavor and spirit than rule and letter) would be Swords & Wizardry Whitebox Edition if you are going by the actual boxed set. There is something about the box itself that just makes it special. The Deeper Delving Boxed set fails to recapture the same magic.
If I want an AD&D feel without the clutter, I'd go S&W Complete. Labyrinth Lord AEC comes damn close, but the need of two rulebooks compare to the one of S&W Complete is the tie breaker for me, if you will.
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The Minotaur for Old-School Essentials
*Requirements:* Minimum STR 9, Minimum CON 9
*Prime Requisite:* STR and CON
*Hit Dice:* 1d10
*Maximum Level:* 8
*Armor:* Leather, including shield...