What is the best professional certificate for Contract Engineers?

MRICS, FIDIC, & PMP are the most recognised certificates in my view.

If you want to become a Professional Contracts Engineer, better take the certificate for the following:

1) Certified Professional Contracts Manager (CPCM)

2) Certified Commercial Contracts Manager (CCCM)

A Certified Commercial Contracts Manager (CCCM)

I would suggest PMP, Project Management Professional as a first step

PMP of course. Project Management Professional

CPP Certification from APS, USA, 

Certified Purchasing Professional (APS)

this the best certificate for purchasing and  contracting 

Certified Purchasing Professional (CPP)The Certified Purchasing Professional (CPP) program is for professionals who have demonstrated the skills to successfully implement improved purchasing and supply chain practices as part of a business solution in an organization.

Contract Engineers are generally entry level positions, wherein they should have certifications pertaining to their area of expertise for which they are seeking emplyment . For instance for Networking Engineer, they should have CCNA certifications and other Cisco certification. Likewise an Engineer seeking a role as server Engineer should have relevant certifications of Microsoft or Redhat, etc.

MRICS, MICOB and FIDIC are most suitable 

International Federation of Consulting Engineers (FIDIC) Certification

Contracts Administration for Engineering

I believe that the best certificate for a Contract Engineer is the PMP.