Different control panels can be used with VPS hosting but which control panel is best will depend on the choice of the users and their preferences. To handle website and email accounts use cPanel / WHM if it is Windows VPS then one can use plesk panel.
if you looking for those value for money and bulk vps control, you can have two choice, SolusVM and Virtualizor. Both of them have similar features that can manage bulk VPS.
In term of storage for KVM, Virtualizor support File Based, while Solus only LVM. Virtualizor support HTML5 VNC, without need to have proper SSL cert, while Solus need. Solus need to manual configure bridge. Solus is Master and Slave based, Virtualizor all node is individual by default, and can be configure slave based. Solus after took over by OnApp, i think their development is less. However, Solus features is more mature and i guess not much need to develop also.
Other then feature, the other important point need to see is stabilities and able to support large VM, SolusVM is in the market longer then Virtualizor, thus it suppose to have more customer then Virtualizor able to demonstrate their control panel can support large VM based.
For paid one I would choose SolusVM, I like the concept master/slave because even the master goes offline the nodes slaves are still working with all VPSs working and no downtime for the users, for the free ones I would choose Proxmox has a lot of features and also you can integrate with WHMCS with a module, Proxmox can cluster but you have to pay for the pro version.
If you want to integrate billing, i would suggest WHMCS and get the modules to control your vps node.
I think this ease the pain of managing multiple system for future growth when your number of customers increase.