The magenta Moldvay boxed set. Hooked me hard into D&D in ’82 and almost certainly changed the course of my life.
Omar Sharif’s 2 5 8. I barely remember how to play this board game, but, as a kid, I was obsessed with its tactical scoring. My gateway into chess, go, etc.
They’ve Invaded Pleasantville and Vampyre. Two TSR mini-games that dominated an entire summer for my sister and me. We took them everywhere with us!
Heroclix. Don’t worry. I’ve recovered from the addiction.
Fate Accelerated. Adjectives for stats? A lite version of a great rpg but uniquely super fun? FAE gave me ways to invite so many more people to the hobby.
Ticket to Ride. All games claiming to be competitive/fun for the whole family failed until we got this one. Played several iterations, and they all succeeded.