What is the building ID that has the most company offices?
SELECT T2.id FROM Office_locations AS T1 JOIN buildings AS T2 ON T1.building_id   =   T2.id JOIN Companies AS T3 ON T1.company_id   =   T3.id GROUP BY T1.building_id ORDER BY COUNT ( * )  DESC LIMIT 1
building ID that has the most company offices refers to the building_id with the MAX(COUNT(company_id));