CN, outside of some of the largest banks, most complaints should be reasonably manageable.
Correspondence from regulators, agencies and governments first, attorneys next, customers then non-customers.
Within each category by product or department. Also a column to indicate if discrimination (or difference in treatment) is noted.
For lending, try to identify, GMI and location, like InFairness indicated. Also, distinguish types of complaints. They ran out of donuts last Friday, or the bank returned my check is a lot different than a claim of discrimination. Some are service issues, not what many banks would consider a complaint
Integrity. With it, nothing else matters. Without it, nothing else matters.