Star Frontiers: in 1987, my Dralasite threw a tangler grenade and thus opened the gateway into the larger role-playing game cosmos. I had the rules, but I’m fairly certain I never played by the rules.
Middle Earth Role-Playing Game: my proxy for Dungeons & Dragons during the Satanic Panic.
2nd Edition Dungeons & Dragons: the game that I snuck into my house, and from which I ran a long-ranging campaign with fellow high-schoolers. This is the game that cemented numerous friendships that I carry forward to this day.
Rolemaster: in 2000, I wrote my first RPG article for the Guildcompanion, highlighting a critical aspect of RPGs—the rules are there for you to hack and extend.
3rd Edition Dungeons & Dragons: a flexible system with lots of player facing technology. More important, an open source rulesset that allowed everyone access and rights to the many layers of the system.
Burning Wheel: a masterfully constructed fantasy RPG that puts the rewards and advancement squarely in the hands of the players. The Range & Cover and Fight system exemplify the idea that combat should be unpredictable and ambiguous for its participants.
Dungeon World: a game iterated on via Github. I committed changes to what would be the final release. I also wrote up several for sale supplements. And last, the breakdown of Principles and Agendas provides a framework for articulating how you approach an RPG system.
Diaspora: I fell in-love with the mini-games, and having played a few space battles using a one dimensional map, I must say that is all I need. Providing a system where out maneuvering a ship is just as viable as disabling the opposition. Diaspora does a fantastic job of highlighting that you need not have a unified set of systems for all elements of your game.
Dungeon Crawl Classics: re-iterated that it can be a howling good time to problem solve with a cadre of fragile characters with nominal resources.
Whitehack: elegant, minimalist without relying on an understanding of the meta-context that is fantasy RPGs.