For me, it was Galaxian, then Galaga. I enjoyed Berzerk, Tron, Star Wars (the Death Star Destruction), and several others. There was one game I almost played as much as Galaga, but I can't remember the name. You had to fly a ship around a space station, shooting at targets on the space station, while the station used point defense and sent ships out after you. Can't recall the name. Early-mid '80s release.
Zaxon?
Star Castle. I have it for PC. It is easily downloadable.