The way I've heard it, the Mets hiring a new GM and front office personnel is not being done in preparation of the team ripping down the roster and going through another rebuild. Instead, the goal is to install people with experience winning so they can build on what exists and and get the Mets back to the World Series sooner than later.
If that's the case, the answer to the question in the headline is simple...
To best way for the team to gain trust and show full-force commitment to winning is to sign free-agent-to-be Manny Machado.