Depends on what area of expertise you want...Accredited Buyer Rep, Epro is great if you need help for marketing yourself or your listings, GRI is highly respected and I recently got my CRS- Certified Residential Specialist, and learned a lot of excellent information. Try to get a new designation every few years.
Your GRI, then ABR. That pretty much covers buyers and sellers and structuring your business. Right now at RREA we are offing the Accredited Staging Professional Designation on Aug 14-15. If interested, use Promo code RREA to register and get $45 off. We'll have lunches provided and you get over 10 MCE credits. Click the link below for more info.
I am a strong proponent of continuing education, not just courses but news sites such as Housing Wire, DSNews and Mortgage News Daily.
I personally dont have any acronyms after my name. I met an agent that had CIPS, CRS, ABR SRS, ALHS, GRI, SRES and ePro on his business card. I thought it silly. I figured this guy is great at sitting in a classroom. He was not very knowledgeable.
The designations already mentioned are great suggestions. I would also suggest CNE (Certified Negotiation Expert). It will help you get through the current seller's market.
They are all important! Always take courses and stay educated. The basic skills are taught in the three GRI courses, and in the ABR and ePRO courses; so start with those.