For me it was SPACE INVADERS on the ATARI VCS. We got it christmas 1981, I had played a couple of arcade games before, something with a mounted harpoon gun on the cab shooting sharks and also lunar lander, but this was the first game I properly played. We got asteroids and Pac-Man later on, interacting with TV was such a big deal back then! Another world behind the screen!!! Also saw TRON in the cinema around that time and that really inspired me with video games.
We later got a Commodore VIC 20, then Acorn Electron, they were both wonderful too, I still have both today. Must try to repair the Electron some day, it has been broken since around 1988.
Although PC is now my main focus for retro(I LOVE to tinker) the 8-bit era will always be the golden time for games to me, I saved for a ORIC ATMOS on sale in the CO-OP just before it closed down, £49 with 12 games, me and dad had such fun playing with the silly built in sound effects, zap, explode etc, I did an awful lot of chores to get that machine, lucky it used the same Saisho tape deck as the Electron.
All the best,
Chris