Taking the top spot in my gravity games list comes Oids for the Atari ST. It's got a touch more substance than most, and even something of an emotional core, concerning itself as it does with rescuing little androids (Oids) under the oppression of nasty aliens.
You have to firstly destroy the buildings they're held in to free them, then land and let them board. Accidentally frying the Oids themselves is a traumatic experience, and is best avoided. Alien spacecraft will take exception to all this and attack you, and there are other things dotted around the landscape that affect you, such as towers that repel or attract your ship.
In gameplay terms that's all there is to it, essentially. It is, however, completely addicitve, fun and satisfying to play. What's more, there's a comprehensive level editor included for you to design your own fiendish worlds to challenge yourself or others with. The graphics are simple - minimal, even, and the sound is barely existent. Which only goes to show how great the game is regardless.