Take Moon Cresta, for instance. It's yet another Galaxian clone, riding the coat-tails of the game that unshackled the alien invaders from marching single file across the screen as sitting ducks, and sent them swirling down at the player like cosmic Stukkas. Moon Cresta would be nothing special, if not for its wonderful sound design. From the majestic opening theme to the squirrelly screams of the dying aliens to the insistent imperatives that pressure you as you play, it's wonderful stuff that really helps fill in the gaps left by the limited graphics available at the time.
Sounds were one of the most important ways of games to draw you in while wandering around a noisy arcade (remember those things?), and Moon Cresta really shoots for the moon in auditory quality. Here's a video to help you hear what I mean.