1. E.T. - The game that destroyed the leading video game company at the time, the game that nearly killed the entire video game industry, and the game that summarizes why these games based on movies need to stop.

2. Pac-Man - One of the oldest real "main characters" in gaming. Pac-Man had personality and was a character, not just an avatar for the player.

3. Super Mario Bros. - This game breathed life back into the video game industry. After E.T. and Atari failed so miserably, Nintendo took a chance and released the Famicom/NES, followed shortly by Super Mario Bros.. This game revolutionized platformers and spawned about a bajillion clones.

4. Enter the Matrix - Enter the Matrix is a horrible game by most accounts, but it sold over five million copies in all! It was a rush job that sold ridiculously well because of marketing alone. That's something that should be examined, in my opinion.

5. Street Fighter II - Revolutionized fighting games. The first fighter to include that much depth and influence so many games that came after. It's even still popular today and many fighting games still try to emulate it's success.

6. Mortal Kombat - Mortal Kombat brought violence in video games to the forefront and showed that games don't necessarily have to be for kids. The game that caused the creation of the ESRB.

7. Virtua Fighter - Renowned for an incredibly amount of depth and pushing the 3D fighting genre, Virtua Fighter is to 3D fighting games what Street Fighter II is to 2D fighting games.

8. Dragon Quest/Final Fantasy - The fathers of the modern RPG. I put them both together because neither of they both played important roles in popularizing console RPGs. One thing to note is that Final Fantasy and Dragon Quest have giant audiences in America and Japan respectively.

9. Tetris - Tetris is still played and remade today. Every falling block style puzzle game is a tribute to Tetris.

10. Galaxy Game/Computer Space/Pong - Galaxy Game was the first coin-op video game ever made. Computer Space was the first video game ever commerically released. Pong was the first one to become popular enough to really call the video game industry an industry at all. They all very important.

I could write a longer list, but I think 10 works. xP