Currently SPG points are the most desirable hotel points. You can use them to transfer to partners as well(20k at a time = 25k at the airline). They also go to marriott at 3x, so 1 spg = 1 Marriott point. Both SPG Amex cards currently have promotions on it, so if you want to start one now, do it before the end of the month. Also, when it comes to marriott bookings, the SPG cards earn more than Marriotts own cards... which is kinda funny in a way...
Hyatt is probably the best when it comes to redeeming. 15k for a $250 room is pretty sweet redemption. 25k sometimes for like a $400 room. Keep in mind though UR does go to Hyatt.
it really depends on what branding you like, there is also Hilton, IHG, etc.