The best dictionary is, as with all best things, very subjective.
I downloaded various dictionaries from
http://download.huzheng.org/ some time ago. Right now this site doesn't seem to work.
If one reads lots of special literature (like for example papers of virology or medicine) with lots of specialist terms most dictionaries are rather useless, though. But then I usually use the Librera app for PDFs and not KOreader, and there I can use the Linguee app (for tranlation into my mother tongue [offline or online]) or Wikipedia.