You need to build consensus around "value" and select a methodology for comparing projects in the product portfolio against each other in a way that is inclusive of all your stakeholders.
The first thing your organization needs to do is define "value" in the context of your group. Value will be defined differently by each organization and each stakeholder, so you need to build some consensus first.
Prioritization can be done in a lot of ways, but in my opinion these boil down to just a couple of broad :
- Prioritize the low-hanging fruit.
- Prioritize based on cost/benefit.
- Prioritize based on contractual or compliance requirements.
There's no "One True Way™" to prioritize anything, but in most cases I think that cost/benefit analysis is the right way to go. If you decide to go that route, then I think that relative weighting is a good approach.
Mike Cohn describes his web-based relative weighting tool this way:
Relative weighting is a prioritization approach that considers both the benefits of a feature and the cost of that feature.
If you have no other drivers, then using a technique like relative weighting might be good fit because it's somewhat quantitative, while still allowing for subjective fudge factors in various aspects of the estimation process.
The real key, though, is to build consensus around a set of priorities that aren't based primarily on political capital. ROI and cost/benefit analyses are often great options for exactly this reason, and relative weighting can really help in that regard.
Your mileage may vary.