Spacewar! (Arcade, 1962)

Back in the early 1960's, an MIT student named Steve Russell and his buddies fiddled about with some old SONAR equipment and primitive computers in their spare time.  What they created was Spacewar! (1962), one of the earliest recorded video games.  This idea was nothing new, talented MIT students had been doing the same sort of thing for almost two decades before, using school equipment to make games, which were typically tennis simulations.  The problem with these games was, well, their limit to MIT students, but Spacewar! was going to change that.
The game can be seen as the first multiplayer deathmatch (in the spirit of Halo or Call of Duty).  Two players control their own spaceships, and try to shoot each other down.  There is a star or black hole in the center which will kill a player that strays too close and gets sucked in.  The gravity effect can be used strategically, however.  With careful maneuvering, you can change your momentum to avoid an enemy rocket.  It used vector graphics, which drew everything in rotatable lines instead of pixels.  The control scheme was complex for the time.  There were four buttons, two for rotating clockwise and counterclockwise respectively, one for forward thrust, and another to fire.  Simple by today's standards, but keep in mind, this was a time when hardly anyone had played, let alone heard, of a video game.
In the early 70's, a brand new computer company called Atari wanted to make their first public video game machine, and they chose Russell's Spacewar!  It was the very first arcade machine, looking like some kind of alien equipment from a science fiction film.  It was a failure, due to the complicated controls which alienated players at the time, although a similar control scheme would be used in the popular Asteroids arcade game.  It was not until Pong that an arcade machine would be largely popular and accessible.
Spacewar! is an oft forgotten game, but its importance in the history of gaming is massive.  Like the early films of Thomas Edison or the Lumiere Bros., it helped define the techniques used in video game programming.  It was ported to many home consoles, mostly 1960's computers that had video monitors, and there was an Atari 2600 port, which suffered to the fact that it had become archaic even to the Atari's standards.  It can be played for free on an online browser today.  Compared to the far more successful Pong, I would say that Spacewar! is superior due to its higher level of complexity and strategy, but then that would be like comparing apples and oranges.