First part of my answer is that I don't think you have to choose just 1 type of user to design for, rather you should have a set of users whom you design for and they should have a clear an well informed priority between them.
A general structure of priorities is:
What is the purpose of this product/service/project? You need to answer this question before anything else. Hint: if you have a purpose, ask "why?" to that. Ask "why?" to that answer. Keep going until you arrive at the answer "to make money". The answer you said just before that is probably your real purpose
Users Who are the users you're designing for? These should be prioritized according to how well they fulfill the abovementioned purpose. For each users you should have a prioritized list of:
Needs What are the needs of the users? This is not "add calendar items to outlook", but should instead be "be able to plan my time". These should be prioritized according to how well they fulfill the purpose as well. Under needs you get your:
Functionality These are the functions that actually make up the final product. This is where you get your "add calendar items to outlook". They are attached to a need, and should be prioritized based on how well they fulfill that need.
So now you have this great big hierarchy of items, at the bottom you have the functions that you need to design for. Make sure that the functions at the top get the most focus, and you're not longer designing for one user, you're designing for your system!
Second part of my answer:
As for which users should be prioritized? Honestly, if the users who make up your main income is not your main focus I'd say you either:
- Have already fulfilled their needs to great extent
- You want to add new market segments
- For some reason profits is not important for your company
Unless you're just doing this for fun, designers as well as every other role in your company need to be aware of the fact that everything you do is either for the end purpose of making the company money, or to enable an entity elsewhere in your company to make money.