For getting my population density up, which of the houses is best?
I've built up several settlements with different sizes of houses to test this. I've found that the smaller the houses are, the most people per area there is room for. Since the very smallest houses are not affected by settlements though, you'd want the second smallest size for any houses close to a settlement.
Building outside of settlements, the smallest houses would be the most efficient.
The trade-off is that smaller houses make collecting large amounts of belief much slower/more tedious, so in the end you may want a mix of both large and small houses anyway.