My #1 would be Robotron--still crazy addictive and gets me completely wired to this day, decades later.
#2 would be Asteroids Deluxe--didn't get hooked until I bought one in the mid-80's, but damn does that game put you in a zen state after lots of practice!
#3 would probably be Ms Pacman--the other pac games are up there, but Miz Pac was the one I remember playing endlessly in bars when in college. I feel a bit of inebriation coming on just thinking about that.
#4 has to be Galaxian--the first video game that really got me hooked. I had played Pong, and Space Invaders, etc., but when a fraternity brother said 'you gotta see this!' and dragged me downtown to the bowling alley to play Galaxian, my video game addiction began right there.
#5 has lots of candidates, including: Lost Tomb, Joust, Space Duel, Black Widow, Phoenix, Pleiades, Gorf....
Strangely (unlike most people here), I was never that into the Donkey Kong games. And I know Defender is a great game, but I've never gotten even slightly good at it (despite having it on my jrok MW cab--Robotron, Joust and Bubbles always seem to take precedent!).
Lots of good choices and memories in this thread....