98 - Galaga '88 (Arcade)
I spent many 10ps in the Galaxian machine of the local chip shop when I was a kid (standing on the stool and all that rose-tinted stuff), and although I never really got any good at the thing it always mesmerized me. A few years later Namco perfected the template with Galaga '88, which I first experienced on the PC Engine (a fine conversion indeed). The core gameplay remains the same, rows of aliens fly onto the screen and join together in a shuffling chorus line, occasionally making divebomb attacks at the player who has to simply clear them out to progress.
There are a few extra things bunged in such as bonus stages (Dimensions, as the game has it) and some modest power-ups, but it's the simplicity of that basic game that keeps it surprisingly fresh and addictive. Some obvious cosmetic additions such as more detailed sprites and a very satisfying range of sound effects are welcome and give it the slight aesthetic edge over previous incarnations.
I'm still rubbish at it, though.