mathematically speaking, the best options are probably the granary, the theater of frost and the boats. Granary and boats give a permanent manpower, and granary and theater add up to 10 more Happiness, so about one more virtual manpower from that.
thematically, I favor theater of frost because we're in early winter, and the new options because the more potential materials for a flute we have, the better. Also, adding chances to attract cultivator manpower will be important, since we want some manning the settlement wall, and the earlier we start rolling those dices with a bonus, the better.
we'll probably not be able to give much focus to the fief in months 13 and 1 since we'll be in Xiangmen for parts of either, so I want to do more "boring" stuff then: granary and settlement walls come to mind.