Super Space Invaders.
Super Space Invaders is to Space Invaders what Galaga '90 is to Galaga - the same basic game idea with immeasurably better graphics and sound and more variety in monsters and monster behavior. Super Space Invaders also has a story line, which is as follows: Unwanted video games have been put into orbiting garbage dumps in the future. One of these falls into the hands of aliens, and some time later, the monsters from the game arrive to invade Earth - for real.
This is the excuse for you to take one of two different vessels (one ground-based, one air-based) and pound your way through dozens of screens of incoming monsters, plus some wicked end-of-level bosses. The UFO which used to drift across the top of the screen in the original Space Invaders is still there in this game, but shooting it doesn't just net you points, it also gets you a powerup icon. Powerups include a row of shields, an extra point for your ship's deflector shield, and various exciting weapons, some of which are just about mandatory to defeat the end-of-level bosses.
Particularly amusing are the between-level bonus stages (Cattle Mutilation scenes). There are a few cows wandering to and fro across the bottom of the screen, and your job is to prevent the UFOs from abducting them. If a UFO succeeds in netting a cow, it drags it up to the top of the screen, and you have a VERY limited time to line up a tricky shot which must miss the cow (struggling in a net below the UFO) and hit the edge of the UFO.
Verdict: You need this game in your Atari ST collection. It's an excellent conversion of the coin-op, and it's a lot of joystick-wrenching button-thumping fun. Don't be fooled by the simplistic theme, it's a very good game.