It's amazing how many people online now have never experienced the original Atari. You still see Space Invaders and Centipede appear in various places occasionally, but I wonder how many people even know how popular these games were in the late 70s and early 80s.
Check out Steve Fulton's article at Gamasutra to reminisce about the days when it was OK for a video game character to have a block for a head, or when things usually moved up and down, right and left but not necessarily diagonally.
There's even a mention of Circus Atari, which was sort of like breakout turned at 90 degrees. I never played this game as a child, but I now own it on one of those 10-in-1 Atari joystick games. The goal is to propel clowns into the air off a see-saw, catching balloons of different values along the way. Circus Atari rivals Dig Dug as my favorite old-school video games.
But man, there are so many good games from the Atari days that it's hard to narrow it down to one or two favorites.