One of the most powerful tools of marketing is third-party endorsements. This is anything coming from someone other than yourselves. It can speak to your effectiveness, professionalism, great customer service, fair-pricing or any other positive attribute. Sometimes all it communicates is their enthusiasm.
The point of course, it’s not just you saying it. It’s someone else and that enhances the credibility.
If you know your customers’ buttons (what motivates them) endorsements that mention or refer to them are particularly effective. “Finally an auto dealer I can trust!” “I couldn’t believe how low their prices were” “Thanks Joe for showing up on-time and doing a great job on my plumbing!”
All of these act as third-party endorsements:

Testimonials
Endorsements (recommendation from some authority, not a client or customer).
Press
Awards
Certificates and Degrees
Acknowledgment Letters
Rankings
Ratings and Reviews

Go for them all. This is an area that almost can’t be overdone.