Main Building RPC.
If you mean which outer building:
Non CL outer building: Big Pine Key (near the beach pool, if that is important to you, as well as the courtyard pool) or Boca Chica - both of these are also quite centrally located. All around good locations.
Actually as Dansdad said, all the buildings are great.
The only two buildings with a few rooms that I might try to avoid are Sago Cay and Conch Key. Sago Cay has some rooms with noise issues early in the morning from delivery vehicles (BEEPING) and Conch Key has some rooms with noise issues form the boat dock (horns). Most rooms in all the buildings are just fine, however. Nothing is far away from anything at the GF. It is very compact.