I'm very happy with Hyatt this year, and particularly with promotions they've ran offering one free night anywhere in the world for every two stays. That promo is not running right now, but may very well be back in October or November. In the interim, they're offering bonus airline miles that could interest you. Just last night I was in a $450 ocean view room at the Hyatt Regency Waikiki using my free nights from this years promos. Other FTers have redeemed their nights at more costly properties in Sydney or Paris.

Hyatt, though, does not have a presence in many countries outside the U.S., so if your travel will take you outside the U.S. it does have limitations and you'd need a backup program. I have Hilton and Priority Club for those cases.