First piece of ark built, either in orbit, or on Earth and launched. Contains command and control, power, environmental, sensors, communications, docking platform and resupply for crew vehicles.
This does not need to be super fancy in the beginning, although for building it to the point where it offers some kind of artificial gravity through rotation is going to be important before long duration stays can be allowed. Crew quarters will be the first structure built, even before the hub of the ark is actually even started. Even if it has to be shipped in parts in multiple launches to be assembled in space. It makes sense for the sleep quarters to be rotating while everything else is in free fall