It's .epub
Use Smashwords for selling epub. They distribute to Apple. Upload an epub2.
Amazon has about 90% of ebook sales. BUT! The best format to upload to Amazon KDP is the same epub for Smashwords. Amazon's advice! Purchased downloads show they dumb it down for old mobi format readers like the DX and do a conversion to KF8/AZW that on the Kindle PW3 looks the same as on a Kobo or Nook. KFX is mostly different encryption and slight rendering differences, so called enhanced typesetting. The epub2 you upload to Amazon is used to generate that also.
So the only thing you need to output for all epub readers and upload to Amazon for all Kindles, is the epub2. Using docx input gives best results.
If you ALSO sell Kindle format via Smashwords there is a slight snag. They don't know which generation of Kindle a customer has, unlike Amazon. So it's the single valid use of Calibre's "Dual Mobi", an old mobi and a KF8 versions in the one file. You upload that to Smashwords, nowhere else and never Amazon, and then when people buy it if they have an old Kindle their Kindle ignores the KF8 part, and if they have Firmware 3.x or later, they can chose "Publisher" on Aa and get the modern part, delivered by Amazon as AZW.