I think it is relatively easy to achieve gold with Priority Club and as others have mentioned SPG will give you gold if you have Platinum AMEX.

Hitting the top tier for each program varies greatly, but I may still weigh in on Priority club being the easiest as they look at points or stays versus some of the other hotels. Priority club also always seems to be running a promotion of some type.

I personally like SPG as the room quality is good while I travel and the hotel locations are adequate for vacation. If your goal is to go anywhere in the world and have your loyalty hotel of choice, I would not pick starwood as the best program.

I sometimes base my vacation around where SPG has properties or I decide not to even bother staying at one.