I loved GURPS. My brother and I played all the time. Also want to shout out Kevin Crawford's Stars without Number if you're looking for a scifi game. He has a free to download basic version, which is fully playable, and a paid version which has some optional rules and tools..
I prefer Basic fantasy for fantasy gaming but Worlds Without Number is also a great option (again free and premium versions)..
The classic explanation (at least how it ends up being used)...