Starwood Preferred Guest
I'm a member of several loyalty programs (United, American, Hyatt, Hertz, Best Buy, etc.) and it's easy to understand why. The costs to joining these programs is usually zero and they reward your loyalty, even if spread out over years, with free flights, stays, or other discounts.
By far, my favorite loyalty program is the Starwood Preferred Guest program. The Starwood Group is an international conglomerate of hotels that consists of such brands as Westin, W, Sheraton, and St. Regis. Their loyalty program has consistently been awarded as one of the top loyalty programs out there and the winning characteristics are availability (no black out dates), flexibility (you can transfer to airline miles, use cash+points) and affordability (free hotel stays for as low as 2000 points).
Combine the Starwood Preferred Guest program with American Express's top notch service and you get one of the most popular reward credit cards out there. $1 = 1 Starwood point.
I absolutely love the program and having only been a member for only 2 years, I've already used it for multiple stays in San Francisco, Los Angeles, Alaska, Hawaii and Canada.
As airline miles get harder to redeem, it's nice to see a worthwhile alternative. After all, when you travel somewhere on that flight, you often need a hotel to stay in. And with Starwood, you have properties throughout the U.S. as well as Europe and Asia. I can't recommend it highly enough.