5. (Classic) Battletech, 1986
I chose the cover of this technical readout because this was the first kind of supplement I had bought for a miniatures game, and memorized it cover to cover.
Battletech was my first minis game, ever. I played it with the same neighborhood kids that introduced me to #3 on the list, and I see a trend developing here. Not only are these my 'foundation' games, but almost every entry has "blown me away" in one fashion or another.
Classic Battletech (we just called it Battletech back then) was just such a game. It was sci-fi, it had minis that you bought and painted, and it used terrain. This was it, this is the game that had me putting figs on the table and going "pew pew" to a complicated set of rules. We would spend whole weekends playing this game, often leaving the table setup to finish the game the next day.