The buses tend to be best at values, there is extensive information on the transportation board about this topic.
We have stayed at all levels of Disney resorts. Of course the monorail is our first choice but that said, going to AK from the monorail resorts takes forever. The Moderate resorts primary weakness, for me, is their bus service. Most of the bus horror stories I have seen relate to the Moderate's. Otherwise I love the Mods. We have stayed at Pop 5 (6?) times (we often split our stay between Pop and another resort) and the bus service has always been excellent, with one notable exception. Because it is a huge resort park close can be crazy, I advise leaving before park close by even just a few minutes or sticking around for a long while enjoying the ambiance as the park empties (or just take a cab).