I'd like to say Villains & Vigilantes, because that would be appropriate for my superhero-heavy blog and love of the genre, but if I'm being honest - and factoring in all the different editions, hacks, clones, reworkings etc - it has to be Dungeons & Dragons.
When I was first introduced to the hobby - although there were flirtations with Top Secret and Traveller - D&D was the backbone of my gaming experience.
And it's what I always end up coming back to. The publication of Third Edition reignited my eagerness to play again, but once I read the rulebook and was horrified by what I saw I found myself heading back to the Old School.
Then later, the publication of Fourth Edition spurred the formation of the Tuesday Knights. It was, however, Castles & Crusades, then Labyrinth Lord, that we actually ended up playing in the early days of our current gaming group.
I have high hopes for Fifth Edition though...