If by "publish for free" you mean publish without paying for the privilege and be distributed world-wide, then obviously it's KDP. You can earn up to 70% royalties on your sales, depending on the ebook's price and other factors. Several other platforms (iTunes, Kobo) also offer up to 70% royalties. (You said minimum, but I assume you meant maximum.)
If by "publish for free" you mean to make your book available without anyone paying to read it, that's a different question. And you would earn no royalties of course.