How much volume do you think you might want on the back top of rack? I have used teh Ortleb 31 liter rack pack (this is a handy youtube video on how to attach it), but I have also had nothing on top of the rear rack except a tent pole bag. And various sized drybags.
Your volume needs determine what you want for that.
I think DJB nailed it with his suggested for bag on top of front rack, something that you can put your valuables in to take in restaurants or stores.