Hyatt and Starwood(Sheraton) are favorite loyalty programs on these boards, and do offer a lot to their elites. The biggest downside to these 2 is their small footprint, which may or may not be a problem to you. If you are primarily traveling to large US cities, you will be fine. If you are going to small towns or internationally, you might encounter problems.

Hilton, Marriott, and IHG(Holiday Inn) have a lot more properties but don't off as many benefits to elites. Club Carlson has been generating some attention recently with some generous program changes, but don't really offer a lot of high end properties for redemptions.

What a lot of folks do, is have a primary and a backup. Pick Hyatt or Starwood as your primary (if one of them works for you), then pick one of the big boys as your backup program. Hilton can make a nice backup program if you are willing to sign up for the Citi HHonors Reserve card to get automatic Gold status. Of course, you might be traveling enough to get top-tier status in more than one program.