Ryan Macklin http://ryanmacklin.com/2012/09/must-play-games-for-designers/ made a post where he asks what three games in your opinion are the ones game designer should play. Go check out the post in link above for details.
My three games following the rules above would be:
Lamentations Of The Flame Princess
LotFP is two things. D&D 3rd edition made more simple or OD&D made more straightforward to play. But there are more. Rules are simple and extra lite. The take on monsters is different as monsters are not meat sacks you beat but unique and strange entities. There is also great Tutorial and good article type chapters from Raggi. There are also several small rules you could say are house rules or improvements but what make LotFP a little different.
LotFP makes old fresh and totally playable straight out from the box. Great read and improvement to old.
My Life With Master
I haven't read or played many indie games with focused rules for its genre and setting. But MLWM is one with a great impact on me. The rules do focus on few topics but you can still use those for almost everything an adventure and roleplaying involves. And there is end game and ending conditions. And it is a short read.
World Of Darkness
For full benefices one should be familiar with old WoD games. In nWoD rules are improved and every splat book uses the same rules as a base for a game. There are some things I don't like that much but the difference and improvements compared to old splat books with rules what are the same but still different is what makes this an important rules book. But without comparing nWoD core book to oWoD rulebooks it doesn't work right.