pong was the first for me. But the most memorable were two games. Radar Ratrace for the Commodore Vic 20, which drove my mother absolutely bonkers. If you've ever heard the theme song you know what I mean. Here it is.
http://www.youtube.com/watch?v=6dzzArqzK4o Imagine that playing continuously for 5 or 6 hours a day every day. My mother was a Saint.
And the other game was Space Invaders. I was so into that game I got an issue of Compute! computer magazine that had the code in it for vic20's, so I typed in the whole game to play it. Then I left my Commodore on for three weeks straight. Because back then the Vic20 I had did not have the tape storage or disk drive. So if you turned it off you lost everything. A storm came through and lightning flickered the power. I was so bummed.