Which architecture for deploying a firewall is most commonly used in businesses today? Why?

In depends on some variables. For example what is the objective of the established network, the actual capacity of the firm that would be developing and implementing the architecture and what is the amount of allocated budget for the firewall system to be adopted.

Screened Subnet Firewalls are most commonly used for deploying a firewall most commonly in business today. This is because the connections from the outside or untrusted networks dealt by an external filtering router and it hence it protects the DMZ systems from external threats via an intermediate security.