I used to joke that Squad Leader was the first RPG I played, because you had a blank leader chit, and there were rules for advancement through playing a series of scenarios. I was strong on the wargamer side, playing in chess club, and my school's chess team; my language skills were poor, so math and chess, universal languages were my friend. Couple that with some slavic abruptness, it was usually best to keep my mouth shut. A sister introduced me to D&D, and AD&D, and from playing Imperium by GDW, a friend bought Traveller and we played that, probably around '79; Traveller was good on long bus rides in chess club for league tournaments, because it had a lid one could roll dice in, and we used 3x5 cards for our characters, the digest sized books were handy too, you got three in the box originally, but you could fit 5 more or something.
I might have played Melee from the The Fantasy Trip first though, memories get pretty hazy. We also played a lot of Snapshot, with Martian Metals minis, on the big FASA deckplans. We would use the big tables in the chem lab, as the teacher there was supportive of our gaming. I bought Gamma World, and incorporated it into D&D, there were rules for doing so in the back of the DMG. Star Frontiers, ran the Volturnus modules, great art, game was somewhat blah. Played Call of Cthulhu, tried Rune Quest, used ICE MERP converted to D&D, tried Aftermath - fitting name for the game. Tons and tons of boardgames, Starfire II was just about perfect, lots of Ogre, GEV, and all the other Microgames, bought a few of the TSR ones like Revolt on Antares, though the Task Force Games ones in the bigger ziplock bags were better. We sort of incorporate all sorts of ideas into RPG's, as it was more sort of Freeform gaming.