There are some good responses here, but keep an weary eye on the cards with an annual fee.
Assuming the Barclay Arrival card, which offers 2% back toward future purchases, and then a 10% rebate on redemption (redeem 10,000 points for a $100 car rental and receive 1,000 points right back in your account for future use): This card carries and annual fee of $89 starting year two. From that point forward you need to spend an incremental $4,450 per year simply to justify that fee over using a card like the Fidelity AMEX which gives a flat 2% cash back with no fee.
Of course you could just open the Barclay card for a year, use the points, then dump it like I did. The $400 signup bonus also outweighs some of this but just food for thought.
I would naturally suggest the Fidelity AMEX (2% on everything, no fee, no cap) over the Cap1 Quicksilver (1.5% on everything, no fee, no cap) but to each their own.