There are a number of options available, and the choice really depends on why you want to pursue a lean six sigma (LSS) certification. Unfortunately,"best" and "least expensive" are on two opposite sides of the spectrum. The top three certifying agencies are ASQ, IASSC and Villanovau. The process basically involves filling an application form listing a few projects where you have performed actions in line with the BOK, studying for the test and then writing the test. The process is similar to PMP.
Many organizations have in-house certification programs; I cannot comment on the market value or effectiveness of all but certain organizations such as Toyota and GE do stand out.
You will also see green/ black belt certification providers such as goleansixsigma and Udemy which offer online certifications at a much lower cost.
It really depends on your motive. If you just want to learn about LSS concepts you probably don't need a certification at all. LMK if you have any further questions.
I'm actually in the same boat. I will be taking the Lean Six Sigma Green Belt course through Project Management Academy at the end of February. While this course includes the certification of LSSGB, I am still considering taking an alternate certification exam. I've researched both IASSC and ASQ. I'm leaning towards ASQ based on what I've read online, but the IASSC is also appealing. I like the fact that they require recertification every 3 years because it helps you to keep a knowledge base. However, ASQ being a lifetime certification certainly reduces costs say over a 30 year career...which I still have about that long to go before retirement. Ultimately, I'm still trying to determine if alternate certification is worth it. I know it can't hurt in the long run, but will it add any value to me currently is the big question. I'm not likely to ever pursue the Black Belt however. Saving Changes...
It depends directly on your current knowledge/experience vs your future career aspirations.
Anyway I would go for an online training, to get tips and hacks on the test, and then apply for ASQ or IASSC exam. Saving Changes...
What is your objective? Are you looking for knowledge or a globally recognized credential?
The reason I ask is that there are other providers that offer their own training and certificate. It's not global, but it's not incorrect information, either. It's also cheaper. There is the risk that if it is not a resume booster, you might be better off reading a book, instead, but you can't really put reading a book on your resume. Saving Changes...