SimplyCash Plus Business from American Express

Cards that are "best for maximizers" are for those who aren't afraid to carry a few different cards to pump up their rewards.
WHY IT WINS
The SimplyCash Plus Business card delivers the most generous rewards on a wide and flexible range of typical business spending, without an annual fee. You’ll earn 5% cash back at office-supply stores and cell phone providers, and 3% in a category you choose, up to $50,000 apiece in yearly spending. Your options range from computing equipment and cloud services to advertising costs.
CAVEATS
You will need to purchase directly from vendors (for example, the airline or the computer company) to get cash-back rewards. There is no signup bonus. Outside the bonus categories, you get just 1%, so switch to a free flat-rate card like Spark Cash Select.
TERMS
Rewards: 5% cash back at office supply stores and cell service providers; 3% back on any one of the following categories: airfare, hotel rooms, restaurants, car rentals, gas stations, advertising, shipping purchases, and computer equipment and cloud computing; 1% back on all else.
Annual fee: $0
APR: 0% for nine months, then 12.5% to 19.5%.