Making up your own is fun, I have made up some of my own systems that have been sucessful with my friends. Otherwise, I would recommend the d20 System, (Dungeons and Dragons, Star Wars), Coda System (Lord of the Rings), or GURPS (which is basically one game with a TON of variants, there are a few basic GURPS books with a whole bunch of different themes, such as WW2, Future, Modern, Fantasy, Alt-History, Space, etc.). Hope this helps.