Of course this all depends on where you are travelling - have spent a number of nights with Marriott, now like others SPG is #1 with Hyatt #2. Marriott is still in the game at #3 with Fairmont at #4.
I personally wasn't feeling the love with Marriott anymore, I found the properties to be good, but not excellent. Most of the experiences were the same, whereas there seemed to be more variety over at SPG and Hyatt.