The best one? Mine... mirtapbx.com
Just kidding, I don't think mine is the best one, but it has some advanced features not found in other pbx, like multi server and realtime. It is a new product, so a lots of features are missing, like reporting, stats, invoicing, graphs, but the most important features are all there.
Right now I have just one client, with 50 tenants and 300 peers. He is really satisfied and he is adding new tenants and peers every day, migrating from another multi tenant pbx.