Shadowrun 4E. Contains everything you need to start a campaign. The point-build system will probably also give you lots of options to play different characters over many months and years.An interesting question I put to my gaming group and which I now put to the Enworld population at large:
If you could use only the core books (PHB and DMG equivalent) for your gaming group, what system would you use and why (I'll even be nice and allow you to use one setting book if needed)?