Although I still have concerns about most Cloud providers in regard to "Vendor lock-in", but no doubt Amazon with AWS did a great job, and pushed the whole market forward since 2006, especially regarding high availability and fault tolerance.
As the biggest public cloud in the world, there is no need to talk about how AWS is doing, but IMHO, best part about AWS it doesn't reinvent the wheel! It has not only used technologies already exist, but even in exams!
Instead AWS directly searches for local partners, it make a deal with company with global testing solutions which is "Kryterion"! (I remember when I had taken RHCE exam, one of examiners was Jordanian, because there are no Red Hat training partner in Jordan!)
BTW, I still love Rackspace more :-D