This question can get a little wonky because some people don’t consider Star Wars to be proper sci-fi. I can see how one might argue it’s really fantasy in space. I also can’t really decided between Fantasy Flight Games’s new Edge of the Empire/Age of Rebellion/Force & Destiny system (does it have a proper name? Star Wars Funky Dice System?) or the classic West End Games D6 system (which is one of my great RPG loves and one of only two games I’ve ever felt like I really mastered; the other is AD&D 2nd Edition). So, I’m going to go with my pick for Favorite Free RPG: Star Frontiers!
I like Star Frontiers so much, I used some of the aliens as inspiration in my Zack Jackson YA sci-fi fiction series. It was the first non-fantasy RPG that really captured my imagination, despite only having played it once in more than twenty years. The system is a pretty basic d% system and some of the skills aren’t quite a relevant now as they were in the 80s, but it still plays pretty well.