The best implementation inheritance hierarchy is none :) If you must, you can use the implementation inheritance for mix-ins / cross-cutting concerns that are

The best implementation inheritance hierarchy is none :) If you must, you can use the implementation inheritance for mix-ins / cross-cutting concerns that are the same for all parties involved, e.g. access control. But even that may be better done with composition, especially when you have an injection framework that wires up certain constructor parameters for you. Where inheritance (extension) properly belongs is the definition of interfaces.