Statcounter.com for me, I use it on several sites.
Easy installation, just put a piece of code in your pages; you can choose to display either their logo, either counter, either to be invisible.
As you required, username/password are required to see the stats, and you can create several users with different roles.
Here is what statistics they offer:
And here is what you can do in admin area:
It's free of charge; only limitation of free version is that it stores last 500 visits - once you have more visitors, the oldest entry is replaced with the newest one. I bypass it by simply downloading the statistics when it approaches 500 visitors.
E-mail reports are also available.