At first I was going to say that we need some sort of dependency graph to focus on what needs to be ported first. As a Plone developer, I see lots of stuff down the stack that needs to be done before Plone could move to Python 3. However, simply ranking based on number of downloads seems to do a pretty good prioritization.

Out of the first 10 packages that haven't been ported to Python 3, I think nearly all are needed for Plone. (Although things like virtualenv are not a requirement for Plone, they are widely used in Plone development)