Mazogs on my godparents ZX81. The game was basically navigating a maze and occasionally fighting spiders. Which was a luck based thing where you just ran into them and waited for the result. Not very exciting but novel at the time.

But it probably wasn't the very first game. I grew up in a tiny coastal tourist town that had four arcades. So it's more likely the earliest games I played were in them but I don't have a defined 'first' memory. They were fairly ubiquitous arcade games so: Space invaders; Donkey Kong; Pac-man; Asteroids; Frogger; Pole position; Paper boy; Lunar lander, etc.

I think the first game that I ever got addicted to (partly because I had the pocket money to sustain the addiction) was Ghosts and Goblins. Been chasing that buzz ever since lol.