I'm not sure how well it actually holds up but I was addicted to Scrapland as a kid. Fairly unique, vaguely similar to GTA. You get to fly around in spaceships and explore, race, and fight with them, upgrading and customizing them piece-by-piece. You also have on-foot sections where you can transform and disguise into the different robot races that make up the population of the city all with their own unique abilities. And a pretty cheesy but fun robot-noir story.