You need to include all email support, phone support and live chat support in my opinion. The customer support is one of the essential issues to run a web host with success.
If you need that support combine the best will be Kayako. It has live chat support, e-mail support and phone ticket. Also it is possible to inform you about tickets via SMS.
For live support you can try http://www.livezilla.net/home/en/ which is free of charge, but I think it is only for live chat (we didn't test this software).
Right, WHMCS for Billing system. However, for Online Chat support, I would vote for LivePerson. The disadvatage to use LP is that it doesn't work with Linux Machines :crash:
If you're doing Shared/VPS hosting then WHMCS is good as it integrates well with control panels and VPS OSs.
If you're more into dedicated servers then Ubersmith is the better bet.
The key is to pick a system that provides the features you're looking for at a price you can afford. Augment that with phone and knowledge base support to meet customer expectations and you'll have a system that's best for you.