The way you have framed this question pre-supposes that a thorough stakeholder identification exercise has been completed. There are always more people affected by a project than you first think. Please make sure you have that first step right.
One useful way to do prioritisation to ensure we are making good use of your time is to demarcate the various stakeholders into the following 4 groups. Bear in mind that one will have to review this demarcation from time to time to see if changed circumstances will warrant any set of stakeholders to be moved to a different bucket. The four buckets in decreasing order of importance in terms of attention to be paid are:
- High power high interest - Always at No.1 [manage this group closely, give them updated status reports, get their inputs etc]
- High power low interest - Can be No 2 or No 3
- Low power high interest - Can be No.2 or No. 3
- Low power low interest - Always No. 4
We have to keep iterating the 4 stage cycle of identify, analyse, prioritise and engage.
Hope that makes sense.
I practice 'systems thinking' to detect the stakeholders. In the time that the system is in design the impact on the stakeholder becomes clear step by step. The art is to see the whole to be sure you have the most important in the picture.
Ruth Malan explanes systems thinking in http://vimeo.com/6219439