Well, these are the banks that support the entire financial system. Refer to the payment landscape explanation. As seen in the 4-party model description, there are 2 kinds of FIs in the payment industry context.  The consumer bank, which has a relationship with the consumer and the issuing bank, which has a relationship with the merchant.

There could be a situation where the issuing bank and the acquiring bank could be the same. Most of the top banks in the US provide both the services.

While I dont want to get into more details of the system , I would like to list the top 5 issuers and top 5 acquirers in the US as of 2011 based on the TPV. Note that both American Express and Discover are included in this category as they are the issuers and the acquirers.

Top 5 Issuers ( 2011 )

The following bubble map represents the top 6 issuers in the US based on the TPV.

Amex is the leading issuer followed by Chase and Bank of America. Discover is the 6th largest issuer.

Top Issuing Banks in the US (2010)

Top 5 US Acquirers (2010 )

Trying to compile this list is very complicated.  Some of the payment processors (such as First Data) provide merchant acquiring services as well and is complicated. A good measurement of unit is the total number of transactions processed.

1. Bank of America is the largest merchant acquirer in the world.

2. First Data Corporation (Was a public company but now acquired by KKR) is the second largest acquirer in the world.

3. Chase (Paymentech) and Citi (Merchant Services) follow the top 2

4. Fifth Third was one of the other large acquirers in the US.

Note: UK based WorldPay (was part of RBS, but now acquired by Advent/Bain) is the largest non-US acquirer.

The following heat map provides the relative size of these acquirers as measured by the number of transactions processed as an acquirer.

Top 5 Acquirers (based on number of transactions processed)