None, personally I have signed the professional tester's manifesto, because I think tester certifications currently do not make sense. Mainly they are to theoretical and pretty useless in the real world if you ask me.
One course that might be interesting is the Certified Agile Tester path. It is one of the only courses that has a practical exam of 2 hours. Here is a review of the course, but I still haven't found the time and the budget to try it and see if is really worth it.
For test automation engineers it could be helpfully to get a programming language certification for example Java or C#. As a tester you are very close to the development team, so having proven development knowledge might a plus.
Blurb from the professional tester's manifesto:
I, as a professional software tester, believe:
That organizations who make money from creating or promoting standards and certifications are biased in their thinking by the
potential financial rewards of convincing organizations that only
certified testers are professional testers. Those organizations may
include those who sell training, consulting or other related services.
That testing benefits from diversity and not homogeneity: that testing
is not a profession that can be standardized but instead needs to
remain an intellectual professional activity.
read the full statement here.