Banks' best customers are its least loyal, according to new research.Roy Morgan Research found that customers in the top quintile (the 20 per cent of customers who account for around two-thirds of the value of retail banking services) give their banks a lower share of wallet than the average customer.Roy Morgan defines share of wallet as the proportion of a customer's banking business that they give to an institution. This includes transaction and savings accounts, loans, wealth management and superannuation.Commonwealth Bank has the highest share of wallet among personal customers. It has a 37 per cent share of wallet among the bottom four quintiles and a 32.1 per cent share among top quintile customers.National Australia Bank has a 34.3 per cent share of wallet among customers in the bottom four quintiles and a 30.9 per cent share among top quintile customers.Westpac has a 32 per cent share of wallet among customers in the bottom four quintiles and a 31.1 per cent share among top quintile customers.Bank of Queensland has a 31.6 per cent share among customers in the bottom four quintiles and a 25.6 per cent share among top quintile customers.ANZ has a 31.5 per cent share among customers in the bottom four quintiles and a 26.9 per cent share among top quintile customers.Macquarie Bank has a 30.1 per cent share among customers in the bottom four quintiles and a 23.4 per cent share among top quintile customers.Bendigo Bank has a 26.5 per cent share among customers in the bottom four quintiles and an 18.5 per cent share among top quintile customers.ING Direct has a 25 per cent share among customers in the bottom four quintiles and a 16.9 per cent share among top quintile customers.Where most banks miss out on share of wallet is wealth management products. Macquarie Bank does well, with a 24.8 per cent wealth share among its top quartile customers, while NAB has a 17.1 per cent share, CBA 14.2 per cent, Westpac 12.1 per cent, ANZ 10.3 per cent, Bendigo 3.3 per cent, ING Direct 1.3 per cent and BOQ zero.Customers in the top quintile deal with an average of 5.7 financial institutions, while customers in the other quintiles deal with as many as 4.7 institutions and as few as 1.9 (the bottom quintile).