Which authentication library ?

[eluser]Colin Williams[/eluser]
The best one is the one that fits your coding preferences (style, performance, data schemes, etc). We don't know what those are, so it's hard to suggest.

Typically, user object models are pretty specific for a given application, so it's always been a stretch for me to subscribe to a catch-all solution. There are a few areas where things are pretty standard, but ironically, these areas usually come down to core CI stuff (like sessions, db drivers, and encryption).

So the short answer is: Write one tuned specifically to your preferred user object model and authorization scheme.