I use Librera on Android. I generally convert to .txt when I read fiction on Linux because I can use a wide range of text editors/viewers that way. It has been a great way to familiarize myself with a lot of features that I don't use when I'm tweaking config files.

Beyond that, I use Okular or Calibre's reader for epubs on an as-needed basis.

LibreraFD from F-Droid. Very customisable, and it lets you use the volume buttons to navigate between pages.

I use Koreader on Android (available on F-Droid or Google Play).

It works. Configuring fonts is a bit confusing — every time I start a new book that uses custom fonts, I need to remind myself how to override it so it uses my prefs. But aside from that, it does what I need. Displaying text is not rocket science, after all.

I used to like Librera, but I had to ditch it because its memory usage was out of control with very large files. Some of my epubs are hundreds of megabytes (insane, yes, but that's reality) and Librera would lag for several seconds with every page turn. Android would kill it if I ever switched apps because it used so much memory. I had a great experience with it with "normal" ebooks though. It was just the big 'uns that caused issues.

LibreraFD and Foliate for the desktop (for epubs).

Zathura on desktop, Librera on Android/GOS. Love Zathura especially. Has all the features I want and none of the features I don't want.

My overall favourite app replacing the Kindle app. It is available on almost all platforms. Fully open source and free.

https://readest.com/

KOReader on my old Tolino with Android.

It's not beautiful but full of features and you can configure everything. I can directly download the books from my Calibre Web Automated container via opds.

KOReader

I use foliate on desktop (https://johnfactotum.github.io/foliate/)

Readest. But I'm looking for a free app with good TTS and (maybe this breaks the free) which is able to handle DRM content from Adobe digital editions (Or is able to remove the DRM). Unfortunately, my library only provide books through ADE.

For Android, Librera from F-Droid. From the non-Google Play readers, it was the best I found.

For the PS Vita, Noboru. Though laggy (lack of RAM in the Vita at fault) and only supporting image-based ebooks, being able to control ebooks by the physical controllers is fantastic, something I greatly miss in any other readers I remember.

@MakingWork I haven't tried many, but Book's Story (from F-droid) is doing a great job in my e-ink phone

I use Librera on phone and Calibre on desktop,or FDReader if I have to open an lcpl.

I dunno about favorite but I use Calibre when I want to read an epub directly on my laptop or some f-droid app (name forgotten) on my phone. Really though I prefer plain luddite text files. I convert epub to text with pandoc, then use either "less -r" or emacs view-mode to read the text file.

Pocketbook on iOS since Marvin is dead.

Calibre for the lappy, Moon reader for the android

Kobo + Koreader

Ebook reader worth every penny. But I don't want to pay big corp

Always used the official books app on iphone.