Selecting Pilot Lean Plant

I'm in a position to select one or two plants out of about 40 in our network that will undergo an accelerated lean / continuous improvement transformation and serve as examples to other plants of working CI/lean process/best practices.

I'm looking for suggestions about what criteria are most relevant in making the decision about which plants are most appropriate. 

My thoughts so far are that we do not necessarily want to work on the worst performing plants as they have issues that could be addressed in different ways by focusing on more basics.  I'm looking at mid-sized plants with a reasonably stable and open management team that has some opportunity for improvement and is ready to go to the next level of operational excellence.

Ideas for the selection process are most appreciated.  Thanks.