No. 6: Tetris.
Russian Block Puzzle Game from Atari
Russian Block Puzzle Game from Atari
Tetris. Atari (1988). Tetris is a remarkable phenomenon
in the games world. So simple, it can programmed by any first year computer
science student, requiring hardware no more complex than your average fridge to
run on, yet it has devoured more hours of more people's lives than both sex and
queuing for a bus, combined. The premise is simple: fit blocks to make lines and
win. The blocks are all made of 4 connected squares, giving 7 permutations and
thus 7 shapes. This is the only real variation on an otherwise rigid conveyor
belt of gameplay. Blocks fall, you make lines. Later levels feature blocks
already in place, or randomly appearing blocks, but really, nothing different
except the speed begins to ramp up drastically. 2 people can play
simultaneously, although there is no interaction except for a points bonus for
clearing a level first. Points are scored by either dropping a piece as opposed
to letting it fall into place, or by making 1, 2, 3 or 4 complete lines in one
go, the latter only possible with a red, long one...
Popularised
by the first incarnation of the GameBoy, Tetris has appeared on virtually every
object in the known universe containing a microprocessor except perhaps Stephen
Hawkins' voicebox. Classic, timeless fun.
The thinking man's Street Fighter. 
Features: The ability to lead heterosexual men standing in a packed arcade on a Saturday night in the middle of Soho shouting "I need a long one!"
Other notes: Yours truly programmed a version of Tetris for the aging Acorn Electron in 1992. Needless to say, this was not a great money-making venture!