I like Ester C.
I was in charge of purchasing for the company that makes Ester-C for 9 years. At the height of our sales boom, we were purchasing well over a million kilograms of raw vitamin C per year. The bulk of it was from China, but a significant quantity was made right here in the USA (New Jersey of all places) and we bought some on occasion from Europe as well.
By the mid-to-late 90's, the Chinese had learned to make C just as well as Mfrs in the West. We tested frequently for all kinds of contaminants, including heavy metals, and the C from China was just as pure as from anywhere else. Because it was fermented in a multi-stage process from corn, by the time it reached its final form (a white granular powder - very small granulation), the only thing that differentiated Chinese from domestic was the consistency of the particle size. Domestic had a tighter distribution.
The reason that the Chinese now comprise the bulk of raw C production in the world is because of unfair competition. They (the "Big 4" Chinese manufacturers conspired) drove worldwide pricing down below actual cost, which made it impossible for higher-cost Western producers (with one exception) and the smaller Chinese players to compete - so they simply stopped making C. When I left the business, there were just the 4 Chinese mfrs and one Western player that had a plant here in the US and one in the UK.