This is an almost impossible question to answer. If online chess counts I would have to go with it.
However, if I had to choose one true video game it would have to be League of Legends. I have been playing since beta, and the game is quite good.
Honorable mention goes to (in no particular order) Metal Gear Solid & Sons of Liberty, Medal of Honor, Army Men 3D, Time Spliters 2, Half-life 2, Fallout 3 & 4, Counterstrike Source, Need for Speed: Most Wanted, Hearthstone, Warcraft II & III, Outlast, Skyrim, Super MarioKart (super nintendo), Mario Bros 3 (nes), Super Ghouls and Ghosts, The Legend of Zelda: A Link to the Past, Rise of the Triad, Zork Nemesis, Roller-coaster Tycoon 2, and Tetris 2 (NES).
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“In the same way that you see a flower in a field, it’s really the whole field that is flowering, because the flower couldn’t exist in that particular place without the special surroundings of the field; you only find flowers in surroundings that will support them. So in the same way, you only find human beings on a planet of this kind, with an atmosphere of this kind, with a temperature of this kind- supplied by a convenient neighboring star. And so, as the flower is a flowering of the field, I feel myself as a personing- a manning- a peopling of the whole universe. –In other words, I, like everything else in the universe, seem to be a center… a sort of vortex, at which the whole energy of the universe realizes itself- comes alive… an aperture through which the whole universe is conscious of itself. In other words, I go with it as a center to a circumference.”~ Alan Watts