What is considered best is subjective, but support for Adobe's ACS4 would offer access to numerous "lesser" ebookstores that collectively could be superior than the Kindle store by itself. (I'm not factoring the DRM disinfection and conversion tools since they are not used, and probably not known, by the majority of ebook readers).
If my reader supports epub with Adobe's DRM I can scout a number of ebook stores using my virtual feet (or the incredibly useful
http://ebooks.addall.com) until I find the title I want. This recently happened to me with a John Varley book that is only available at Sony's ebookstore.
Just a note: I'm not Kindle bashing. That would be foolish considering the thousands of happy Kindle owners out there.
If I'm correct in my understanding that Kindle firmware updates can be pushed wirelessly, I wouldn't be surprised to see epub support on the Kindle in the not so distant future either with or without ACS4 support. If selling Kindles is more important that selling ebooks, Amazon could strike a competitive blow by supporting ACS4. For many ebookwise users the lack of support for Adobe's DRM monopoly is a reason not to by a Kindle.
I should've bought stock in Adobe.
