PAE: Assembly and flight testing for 747, 767, 777.
RNT: Assembly for 737, 757.
BFI: Flight testing 737, 757, flight development for all models (widebodies rare).
MWH (Moses Lake): Very limited flight testing all models.
In terms of 'action' BFI will be the most busy, although that's a relative term - maybe 3 or 4 active on any given day.