There's a current Twitter/Facebook meme, #RPGDNA, where people post covers of the four games systems that made them the roleplayers/gamesmasters/designers/writers they are today.
Here are mine, with a bit of backstory:
Holmes Basic Dungeons & Dragons: After I was introduced to roleplaying games I wanted to learn about them for myself and so went to The Dark Tower in Tunbridge Wells and blew my pocket money on this book. And the rest is history.
The Arduin Grimoire: I have no idea what prompted me to buy this trilogy of books from a small ad in the back of an early issue of White Dwarf, but I'm so glad I did. I'd been introduced to gaming by a Tolkien fanatic, and so that flavoured my perception of the game... until I encountered Arduin. Not only did I discover the concept of rules hacking, homebrewing, and houseruling at an early age, but I also realised it was more than okay to make the game your own.
Villains & Vigilantes, Second Edition: Having always had a passion for superhero comics, I picked up the first edition of Villains & Vigilantes from The Dark Tower, but never really got into it. However, with the publication of the (almost entirely overhauled) second edition, I dove headfirst into this game, thanks to the inspiring gamesmastering of Steve. This led to the creation of our global, postal superhero game - HeroPress - and I think you know where that led.
Traveller: One of the first games, after D&D, that I was introduced to and I always loved its elegant simplicity and the fact that creating a character was an adventure in itself. As a tween and teen, I remember spending hours creating my own character professions, sector maps etc Sadly all now lost to the landfill of history.