Great question Kate!
I decided on the Online Consortium's Instructional Design Certificate. It was a four-course sequence, which I found very beneficial because it introduced a lot of resources in the field and connected me with other instructional designers.
That is before I knew anything about ATD, which has its own ID certificate.
Those are the two I know of. They were better for me because I didn't want to pursue another master's degree so they scoped right for me.