1.PONG: First game to get non-computer geeks to play!
2. PAC-MAN : First COMMERCIAL Success, Space Invaders was big
but it did not inspire CARTOONS, TOYS, Etc...First game
to attract large numbers of women, too!
3. Pitfall/Super Mario Bros. Pitfall introduced this genre but SMB refined it by adding identifiable cartoon characters. Pitfall harry was just a stick-man, MARIO & LUIGI were like Pac-Man, markatable! Sold the NES and brought games back; inspired games that continue to this day!
4. Tetris: First hugely successful COMPUTER game/PUZZLE game. Turned millions of non-gamers into ADDICTS! (:
5: Castle Wolfenstein 3-D/DOOM: First "3-D" FPS games. First blood and guts/ gore games. Began the "3-D" revolution that continues to destroy all fun gaming (personal opinion)

Those are my picks. We welcome yours.