SPACE INVADERS PART II / Taito / Arcade
Space Invaders Part II isn't really so much a sequel as it is designer Tomohiro Nishikado finally getting the hardware to do exactly what he wanted about a year after release of the original Space Invaders - adding color, having the monsters descend onto the screen in a scripted animation, and having a little cutscene of a fleeing monster between levels. This is also the game most people are probably thinking of when they talk about "Space Invaders", as opposed to the monochrome original which kind of got phased out of arcades after this came out.
Part II's only major change is amping up the difficulty a bit by starting the aliens lower than they were in the original game right from the beginning. While perhaps better for hardcore vets of the first game, this is a detriment to the more casual player, as the game's levels become unmanageable much faster leading to much shorter play sessions.
* Gameplay Video