It really depends on upon your requirements. SolusVM is now opened by OnApp and is now from quite a while working for the new 2.0 release. The new release would probably be way better than most of the mentioned panels here.
From a business point of view, SolusVM and Virtuozzo are two good products to go with. If you need a free alternative then Proxmox is not that bad either.
The OP has his business listed on his profile as ispsystem.
__________________ Hostirian - Saint Louis Data Center and Managed Services Provider (800) 615-9349
Datavault, Colocation, Bare Metal Servers, Cloud Servers & Managed WordPress Solutions
Over 18 years of experience working with small to medium sized businesses.
We run Proxmox for a number of our customers have have been happy with it. The underlying virtualization is technology is all open source, KVM and LXC. Proxmox wraps a bunch of open source software into a nice, easy to use gui.
Their paid subscription provides support and easier upgrades.
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