Well the best reward to get is probably a Gold Foil Djinn Oshannus. It gives 12,500 Collection Power, which would bring you almost to Silver III and it is after all a very useful card! The best reward I got was probably a Djinn Chwala or Djinn Renova. Also like the Lava Launcher. If you get a bunch of DEC (biggest one I got was once around 200 DEC), since you could spend it to buy any card. But usually it is better to get a card, than DEC or even credits.

@andy-plays