Currently I’m using booky mcbookface but the app is really lacking and the scrolling is not smooth.
Edit: I came across Lithium and Readera which both don’t seem to collect much information compared to other apps. Not sure if they’re open source though.
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.
I’ve had the large file issue with Librera too. Bundled epub collections with absurdly large page counts have sometimes been extremely slow to load. I’ve had better luck recently, so it might be a partially solved problem.
Good to hear. For context, I made the switch late last year, so my experience may be outdated.