Traveller (the LLB's), I suppose...however...
I started with D&D in early '75, had a lot of fun mixing D&D with Tunnels & Trolls the next couple of years, and then Traveller came out in '77. To be honest, I moved almost entirely to Traveller at that point as the group I played (and GM'ed for) were much more into Science Fiction than Fantasy. Over the past 30+ years I've flirted with Twilight2000, Aftermath, GURPS, Fudge and several more systems, but I always come back to something Traveller-like for SF and something D&D-like for Fantasy.
So, I'd have to say Traveller (in all versions and heavily houseruled) is really my first love, while D&D (again heavily housefuled) is my second.
I started with D&D in early '75, had a lot of fun mixing D&D with Tunnels & Trolls the next couple of years, and then Traveller came out in '77. To be honest, I moved almost entirely to Traveller at that point as the group I played (and GM'ed for) were much more into Science Fiction than Fantasy. Over the past 30+ years I've flirted with Twilight2000, Aftermath, GURPS, Fudge and several more systems, but I always come back to something Traveller-like for SF and something D&D-like for Fantasy.
So, I'd have to say Traveller (in all versions and heavily houseruled) is really my first love, while D&D (again heavily housefuled) is my second.