Which Version of MS 2008R2 to use for Hyper-V

Apart from server consolidation some of the other areas where you can use HYPER-V are,

* Test and Development
* Server Consolidation
* Branch Office Consolidation
* Hosted Desktop Virtualization (VDI)

Microsoft free HYPER Visor is good option for testing and R&D. If you are planning to consolidate more than 4 servers in one physical server then moving to Data center version will do huge cost saving to you.