I think I've tried every ebook reader available on iOS.
KyBook3 is a very capable program. It's continually updated. It offers the most control over my reading display--I can highlight and copy everywhere. For epubs/mobis, I can set the fonts I want, create "styles" for reading (so apply-with-one-click small font for poetry vs. larger for prose). It reads every format I've ever seen. It supports
iOS 6.0 and later. It does a lovely job pulling files from any cloud service (except iCloud, but I think that's iCloud's fault), as well as offering a built-in web browser.
The only drawback for me is
development and dev discussion is on GitHub. I've never received a response to an email, and I don't speak GitHub. KyBook3's developer is clearly a coder first, and Kolyvan's replies to GitHub messages are often argumentative.
Nonetheless, I've happily been throwing money at Kolyvan every 6 months because I appreciate what's there now.