If you are a southwest flyer and want miles for southwest flights it's pretty easy to get a couple of the Chase southwest cards and qualify for the companion pass

If you are looking to travel internationally with your points/miles then I'd suggest the chase sapphire preferred for chase ultimate reward points, the Amex everyday preferred for Amex membership rewards points, or the Starwood Preferred Guest Amex that earns SPG points that have a very nice airline transfer rate.

The card I am using most these days is the Citi Prestige card. I travel almost exclusively on American and the Citi Thank You points I earn redeem for 1.6 cents per point on American flights. The card has a steep annual fee but its easy to recoup if you use the benefits.

So much of your decision depends on how you spend and what you want to do with the points.