Good question, i think one of the very first companies to become known globally was liberty flights as they ship to the uk and throughout the states. Liberty flights i would say is definately the biggest.
No, just like all "retail" suppliers, Liberty Flights is just a reseller. Runyan, who invented personal vaporizers, is at least one of the biggest. As far as "retail", unfortunately, the companies like Blu who provide "bottom of the barrel" models probably has higher sales but that only means what we all know: they put 95% of their money into marketing and not into providing a better product.
In the ECF world of individually owned suppliers, three compainies that I believe are in the top 5 % would be:
Liberty Flights
Nhaler.com
madvapes