I was a big fan of Moon Reader but have changed over to Bookari (both under Android 8.1) because of it's EPUB3 support. It's an excellent tool for previewing and testing upgraded book files with full backward support for EPUB2 so I can check that they work well when released.

Guys I use aldiko reader and a premium one. You can get it from aptoide market and it`s one of the best I have seen. I hope this helps. However I don`t know about iOS based e-readers as it isn`t the same platform like that of an android. I am sorry for my English. Hope you understand.

ReadEra is my favorite

I'm also still searching for a reader but I found that with some Apps you just need convert your ebook. it's a bit tiresome but it work sometimes.