If you have no knowledge of the circuit, a replacement transistor should meet or exceed all the specifications of the transistor it is replacing. If you feel confident that you understand the circuit, the replacement only has to meet or exceed the requirements of the application. This is because seldom is a transistor a "perfect fit" for an application, instead, it will meet the minimum requirements of every spec, and exceed it in others. Thus the second option allows for much greater flexibility in choosing and sourcing a replacement.
There is no absolutely reliable rule about which ones are important, because that depends on the application.