At this point, Hyatt would be your best program as they offer 1 free night for every 2 paid stays (not nights mind you) until August 31 at least. You may also be able to get in on the 50k promo with Hilton HHonors for 4 stays during a 90 day period.

Unfortunately, Hyatt is not always the best priced or lacks in locations and variety of places. Best to have a backup program like Hilton or Starwood. Can't comment too much on Marriott as I had always been disappointed with their program and found the rooms to be very boring. That being said, do the Hyatt promo for the free nights you can still use after August and do the other programs depending on your preferences- Hilton allows you to double dip with airline miles and Starwood offers great 500 points per online booking and has better conversion rates of points to miles although many of us have visted the Hilton Cleaners to laundry miles.