The best for strictly server billing is UberSmith. Why? Ubersmith Professional comes with several nice features - one being they provide device monitoring as part of the script so your clients can view it. You can setup as many monitors as you want for as many machines and specify which client sees what.
While I'm probably not fully "qualified" since I haven't been using it too long, my vote currently goes to UberSmith as well.
I've gone from WhoisCart to ModernBill and now to UberSmith. ModernBill did almost everything but at times wasn't the easiest to use. I actually just installed UberSmith this week and so far am very impressed.
Uber is almost designed more for dedicated billing in my opinion than hosting. Its a great product.
Lpanel. It can now handle billing for dedicated servers.
LPanel is a decent product but really needs to work on support and improving their order form to not show table borders in firefox and other popular browsers. They designed it with IE in mind. Strange.
Will have to look at Ubersmith, to be honest I don't know that I will use a MB product again. The amount of work it takes to keep their product running smoothly just isn't justifiable IMO. I have used AWBS, but won't be offering hosting with this new venture just dedicated services.