Which part has the least faults?
SELECT T1.part_name FROM Parts AS T1 JOIN Part_Faults AS T2 ON T1.part_id   =   T2.part_id GROUP BY T1.part_name ORDER BY count ( * )  ASC LIMIT 1
least faults refers to MIN(count of faults); part with least faults refers to part_name with MIN(count of faults) from joined Parts and Part_Faults tables;