Probably workshops, so you can get more blue cowries sooner and in turn be able to upgrade various things sooner. Granary/Workshop III is the final stage of each, they simply do more of whatever they do. Workshops earn you blue cowries, granaries get gathered items for you automatically. Each upgrade also gains you exp, so you want all the building spots occupied and you want them all upgraded(you can make three workshops and two granaries). The bathouse is just for flavor. All the landmarks are. It doesn't matter which ones you make(you could make four bathhouses if you wanted), just that you build a landmark on every landmark spot. The reason for that is each one increases your groove cap. Groove increases workshop profits. Another important thing about granaries is that they bring back one special item type from each island area that you can't gather yourself, and those special things are required for all the most profitable items the workshops can make.