It depends on what your spending needs are..
If you spend a lot in travel (or potentially) .. your zync also has a pack for travel rewards or Citi Thank You premier is attractiveBest Travel Rewards Credit Cards With Free Signup Bonuses
If you eat out a lot .. your zync also has a pack but the Citi Forward also has a nice rewards package for that as well
If you're looking to earn cashback from the companion and just accumulate points on your zync theres the Discover More which offers 5% in "Categories" (that part bothers me) and then Amex Blue Cash offers 5% on gas after 6,500$ spent.Best Cash Back Credit Cards For 2011
There's a ton of options it just depends on what you need next/your spending strategy, but those are the top suggestions in each category.
best of luck !