I would suggest Amex Preferred Rewards Gold or Everyday Preferred. Both have the best earnings potential (IMO) for a typical family that purchases gas, groceries, plane tix, etc. Amex points can be transferred to carriers in all major alliances (sky team, one world, star) to allow for quite a bit of flexibility in terms of travel options.
I've had good luck with the Starwood card as well, but there is no guarantee the 1:1 transfer ration of Starpoints to most frequent flyer programs will continue if the Mariott merger goes through.