Robert Zoellner is consider to have been the first to do so. His collection was sold by Siegel in 1998. At that sale, Mystic bought the 1c Z Grill for close to $1,000,000. A few years later, Bill Gross traded his plate block of the inverted Jenny, for which he paid $3.4 Million, for that stamp, completing his collection.
The catalog of the Zoellner auction is on the Siegel website at:
http://www.siegelauctions.com/dynam...?sale_no=804 You can download the entire 400-page catalog in .pdf form on that page. It is a great reference, as it includes introductions to each of the different issues, descriptions of each stamp, and census information on the rarest of them (as of 1998). Siegel updates the census information on their website on an ongoing basis.
The introduction to the catalog, where Scott Trepel tells the story of the assembly of the collection can also be read online at:
http://www.siegelauctions.com/1998/...os/intro.htmSo, the answer to your question is 2 - Zoellner and Gross (though some may add Miller, whose collection in the 1920s was considered complete at the time).
Edited to add: Catalog can also be found on
ebay:
http://www.ebay.com/sch/Publication...nkw=zoellner