For my spending profile, I come close to maximizing my rewards with:
1. Citi Driver's Edge for 6% at Gas, Drug Stores, and Grocery Stores, plus the Drive Rewards program pays 1 cent per mile driven (which can double your 6% earnings to 12% in these categories).
2. Citi Professional for 3% at restaurants, vehicle rentals, home improvement, and office supplies.
3. Fidelity Signature Rewards Visa for 1.5% back on everything else.
That's actually the best mix of 3 cards for my spending profile suggested by the rewards calculator at:
Credit Card Tune-Up: Discover the Best Credit Card Reward Deals