I'll suggest Traveller, either the Mongoose Publishing edition or the Classic reprints. It is a (relatively) rules-light interstellar sci-fi RPG inspired by 1960's-1970's sci-fi classics such as Asimov's Foundation and Niven's Known Space. The rules are quite modifiable so you can easily adapt Traveller to a wide variety of sci-fi settings as you desire. And even with the core alone you can play interstellar mercenaries, space traders, space pirates, interstellar explorers and many other sci-fi adventure campaigns.