I see a lot of questions from user interface designers about the "best" certification for designers. Truth is, there isn't a highly regarded certification for UI designers. I know that a private company, Human Factors International, grants a "certified usability analyst" credential, but it looks to me like an entry-level certification. It doesn't account for experience or the ability to actually design a user interface, which is documented in a portfolio.
A better certification for designers is the PMP certification from the Project Management Institute. One of the knocks I hear against a lot of designers is that they don't know how to work projects. Studying for and passing the PMP test addresses that concern.
It's important for designers to understand how to navigate projects in order to deliver their service effectively. I worked a company that offered free PMP training and paid for its people to take the certification test. The company understood the importance of having all of its people know how to operate in projects. Unfortunately, my supervisor at the time didn't understand. "You can't take that training, it doesn't have anything to do with your job," I was told. I took the test some years later, and wished that I had done it earlier.