Metal Gear original boxart (source)
Metal Gear was the first game of the Metal Gear Solid franchise. It was released in 1987 for the MSX2. It is seen as the game that kickstarted the stealth videogame genre, in which the player is encouraged to avoid confrontations with the enemy.
Metal Gear gameplay screenshot (source)
In the game you must take control of Solid Snake, a special forces soldier working for the group Foxhound. As Snake you must infiltrate the fortress known as Outer Heaven. During this infiltration Snake encounters many bosses, including Metal Gear, an armoured bipedal tank with nuclear capabilties which Snake must defeat to progress.
The game was a huge success, and was later ported onto various other systems including the NES, PC and Commodore 64.
The 2008 Edition of the Guinness World Gamers Records crowned Metal Gear as the first ever stealth videogame.