I've used Whoiscart, WHMAutoPilot, and ClientExec. Of the three, I prefer ClientExec with Whoiscart running a close second. But truly it all depends on what you are trying to accomplish in your business.

If you want everything automated and you are chasing the cheap hosting clients, then just about any billing software will work for you. For the first year that I ran a webhosting business, I didn't even use billing software. I collected a check from my customer for the first year of hosting in advance.