What's a piece you yearn to play? Follow your curiousity... If you don't have pieces in your head go explore the unfamiliar - hit the library for recordings, buy some random disks if you've got cash to spare, or just pick up some sheet music of something that strikes your interest but you've never heard before, it's an adventure!
Or maybe you mean to learn a piece that will further your technique, in that case the question is what weakness you would like to approach? I'm always aware of my weaknesses (because there's so many, hehe), that makes it easy to always know what to work on.
That's the Cm partita right? I love the rondeaux from that!