I use the
BISAC subject headings, made into hierarchical tags. It took some setting up and a little bit of annual upkeep since they update the entries annually, but it works for me. I use dummy "books" with all the tags (one for each major subject heading) and don't allow any extraneous tags in the default "Tags" field.
I also have a secondary tag column that I use to build other columns out of. That's where I track the epub version number (if there is one), edition number (if there is one), regional version, if I have the audiobook, etc. It's also where I track temporary to-do tags for my processing workflow.