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.
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.