I’ve tried many alternatives over the years, but in the end I always preferred JabRef (I had multiple “tries” so to say). The reasons are simple:
a) Built around bibtex/biblatex: JabRef is pretty much built around bibtex/biblatex database files. For all the alternatives I always had the feeling that those are cumbersome, because those were not built around bibtex/biblatex database files, but the functionality was just provided additionally. In my opinion, in the details that shows very much. In JabRef it’s e.g. easy do directly modify the underlying bibtex/biblatex code, and that code is also pretty much directly translated to/shown in the UI. There is nothing “in between” the code and what you see in the UI.
b) Groups: I heavily rely on grouping of entries (have >1000 entries in my files…). JabRef still seems superior to all the other options here for multiple reasons.
c) Unusual features: I guess I also rely on some more specialized features in bibtex/biblatex, like non-standard fields/tags, automatic filtering by those and/or attributes of entries, cleaning, conversion, etc. I always got into trouble with those additional things in alternatives to JabRef, which JabRef can handle them pretty much without any troubles.