Indeed, your choice of an app for reading ebooks depends on many factors, some of which are mentioned above.
E.g., the language you prefer reading books, your book shopping/borrowing habits (Amazon, Gutenberg, Archive.org, Libgen, etc.), format of books, even your country of residence, however strange this might sound...
For years, my all-time favourite has been
Moon+ Reader Pro. It can read almost all well-known book formats, including FB2 (which is very simple structurally, yet widely used in publications in my part of the world, although I sometimes prefer the eye-candiness of EPUBs too). Recently, after I encountered a few problems
described in another post, I tried
Pocketbook Reader as well. I can say that it is as powerful as Moon+ Reader, but in some aspects even exceeds it (e.g., it didn't cause the problems I ran into in Moon Reader, and you can use it for listening to audiobooks too).
For reading on my desktop computer and to view and test the way my own creations look on the screen, I use Calibre Viewer, which is part of a bigger suit called
Calibre, and Kindle Previewer!