There are many crucial point to put in mind to design an opamp, but it is based on your design requirement!
Try to list your design spec 1st (example:Av, f3db, GBW etc) and then try to reach the target! Then you will find what is the crucial point in your design.. :lol:
if you build high quality HiFi-circurits or low level
signal for example, you need calculate signal with
noise-flor from first OP-amp stage and so on...
It all depend on the specification that you are looking for. Most of the time the process we use to design a opamp will be the crucial point as it will limit the design parameters and the rest as others mentioned in the post.