What’s the best plug in for having multiple bloggers on one WordPress website?
It really depends on what you're looking for. There are so many plugins for different features. Just to name a few, there is Role Manager which allows you to control roles and access different authors on your blog. There is Adminimize which gives you the power to assign rights to certain authors. There is also SP Authors which allows multiple authors to be assigned to a Post or Page.
The possibilities are endless and it really depends on your specific needs.