A Great Original must be either a video or a computer game that creates an entirely new type of gameplay, or it must be the first successful game of a type to make a given play mechanic work. Preferably, they should also continue to stand up to this day.
(By no means is this an all-inclusive list. More will be added later. Please send any corrections via primate msg through Chatterbox.)
Rogue
Pioneered the Roguelike genre. Invented randomized dungeons and the “identification” item system.
Tetris
Started the “Falling Blocks” puzzle genre, and much copied even to this day.
Colossal Cave
Everyone knows this one. Colossal Cave was the original text adventure.
Zork (or maybe Dungeon)
The first text adventure that allowed the user to converse with the game in full sentences.
Lode Runner
One of the great early puzzle games. Was the first game to offer the user a level creation utility.
Wizardry
The first explore-the-maze computer role-playing game, these types tend to stick close to their Dungeons and Dragons roots. The Bard’s Tale and the Might and Magic series owe a lot to this game.
ToeJam and Earl
Took Roguelike concepts and applied them to a hilarious console game, but is really special for its two-player mode.
Super Mario Brothers
Originated many concepts that are now taken for granted. This was the first scrolling run-and-jump platform game. (Also known as a guy game.)
The Legend of Zelda
The first game to successfully marry the concepts of an action game, an adventure game and a roleplaying game. Originated the concept of making the player search a large world for secrets to raise his “level.” (Heart containers in the original, Pieces of Heart in later installments.)
Adventure (Atari 2600)
The first video adventure game, period.
Gauntlet
The great arcade slash-fest. Extremely well-designed and allowing up to four players to fight at once. The idea of enemy generators was a creation of genius.
Rampart
An utterly unique combination of Missile Command and Tetris that, when considered at a distance, is really nothing like either game. Has a unique three-player mode in some arcade configurations which overshadows even the brilliant solo mode.
Herzog (MSX)
Few remember this one. Predates Dune II in the real-time strategy genre by some years. Beloved cult favorite Herzog Zwei, for the Sega Genesis, is better known than this and also predates Dune II.
Starflight
In addition to having a storyline that qualifies as real science-fiction, Starflight used a fractal algorithm to generate the galaxy star map and planet surfaces on the fly.
Karate Champ
Karate Champ was the first person-to-person fighting game with an emphasis on competition between actual human opponents as opposed to CPU-controlled drones. It is the predecessor of Street Fighter and its innumerable clones.
Wolfenstein 3D
It may be predated by Softdisk's Catacombs (programmed by the same people), but it was Wolf 3D that really showed people what was possible and created the first-person-shooter genre.