The way I've heard it, the Mets are not hiring a new GM and front office so they can rip down the roster and go 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's owners and new administration to gain trust and full-force commitment to winning is to sign free-agent-to-be Manny Machado.