WHMCS would be my vote.
Modules have been written for most of the main stream hosting apps such as WHM\cPanel, Direct Admin, and SolusVM just to name a few, but also does task such as registering domains via Enom and a few others, purchasing SSL certs, setting up applications for a website via Softaculous during the purchase process, etc.
Plus it is in active and constant development and has good support.