The most common billing system is a2billing. which is outside the scope of FreePBX. But it's like me asking what is the best car for my current situation.

You've provided no list of needed/required features, what you want it to do, why, what are your objectives, and the list goes on and on. If you are thinking about multi-tenant situations then know up front that FreePBX is not currently designed to support that. You can google this site (or use the search on the top right) to get more info on it. But everybody who goes down the multi-tenant ave at this time has to do customizations to make things work.

So it's hard to answer a open ended question, thus the use google response which does work when you provide enough key words with specifics you need.