EPUB is a common ebook format around the Web, but the Kindle can't read it natively. That's OK; you can convert .epub files to Mobi files for the Kindle to read. The key is a free piece of software called Calibre.
Similarly one may ask, what is the best format for Kindle Paperwhite?
AZW is the Kindle format, so most files you download from the Kindle Store will have an AZW extension. Note that some files might have a number, such as 1, 2, or 3, after the AZW. MOBI, a file format developed by Mobipocket, is widely used and compatible with the Kindle Paperwhite.
What file format does Kindle Paperwhite use?
You are limited to the types of files supported on the Kindle: at present, these are .azw, .azw1, .txt, .mobi, and .prc. In addition, Kindle can read .pdf files (but these keep the pre-formatted paging and layout of the original file, rather than the dynamic/flowing text of an ebook.)
What is the most popular format for ebooks?