EMCHATEAU  For me also the best i've tried (and i've tried almost all of them). Because of the mac interface, it's a really easy soft for mac users. But it is also very powerfull and truly manageable.

I really enjoy the keywords system and the smarts collections. PDF meta data are easy to receive and it makes it really easy to add a new ref from the pdf on your computers.

Compared to Zotero, it's really more powerfull and easy to use but it doesn't have the export of library on the web and share collection that zotero gives.

Anyway the data are exportable in different formats so you can go from one system to anotherone witch makes your data really safe.

I regret it doesn't support different language for the date citation as i'm french. But i hope they'll change this very soon.

As i'm an historian, i also miss the circa citation in the date field.

Even of that, i'm still using it, because it's so great !  
(Version 5.7.5)