Star Frontiers by TSR. I got the original boxed set, the Alpha Dawn reprint, the Knight Hawks space combat expansion boxed set, and finally Zebulon’s Guide to Frontier Space (a softcover rules expansion/reboot that never went anywhere) I spent hours building out my homegrown star system of Starrior, its own megacorp (Astro Mining and Freighting aka AMF), and dozens of star fleets.
What I did not do is actually play the game more than once or twice. As a result, it easily has my highest ratio of “game prep time” to “game play time” of any RPG I’ve ever owned, but I had so much fun building those worlds, fleets, and potential stories that I’d never consider that time to be wasted
In my 2010 column Game Day: The Star Frontiers Legacy, I wrote about Star Frontiers and its influence on my Star Wars campaign. If you’d like to learn more about the game, check out Tabletop Tribe’s review: Star Frontiers: D&D’s Long Lost Sci-Fi Sibling