I have stayed at all four of them and we generally don't rent a car. So I have used the busses quite a bit during our times at WDW. Here are my thoughts:
CSR generally has very good bus service since many of the guests of that resort attend conferences and don't go to the parks often if at all.
We had wonderful bus service at CBR, it has seven stops total, but we never had to stand going to the parks during our nine days. I really was pleased.
POFQ is first to be picked up and dropped off when it shares busses with POR.
And last but not least, POR was fine when we stayed there. Although as another poster said, by the time it goes to FQ then makes the stops at POR, the people at the last stop might have to stand. During our Christmas week trip when we stayed at POFQ, there were many times the bus couldn't even stop at that last stop at Riverside but the driver radioed and told them to send another bus which hopefully happened very quickly. Again, when we stayed at POR we didn't have any problems.
All this said, I think bus service can be hit or miss at any resort. I have always had good luck at all the resorts we have stayed at except Boardwalk which was a bit of a nightmare. I think you just have to go in and be prepared that you may spend some time waiting and standing! And if you don't have to, its a nice bonus!
To be honest, I never base my choice of resorts on the busses, because its just really too hard to predict.