Having used, extensively and in anger, Hyper-V 2008/R2/2012, VMWare ESXi 4 thru 5.5 and (Citrix) XenServer, for Windows and Linux VM's, my personal favourite is XenServer.

Hyper-V works, but has terrible admin tooling, especially if not on a domain

VMWare is a fine hypervisor and if you are happy with what it provides then ESXi free is hard to beat

But if you want a cluster on the cheap, with proper clustering support, XenServer is hard to beat. I use this extensively especially in SMB and also local dev environments - a couple of Xen Servers backed by fastish NFS SAN's. Easy to maintain and administer. Good tooling both from a GUI and CLI point of view, without actually having to dish out any money.

That said, if someone else was paying, ESX would be just as good if you needed vMotion/vStorageMotion etc.