Visualization wise, which product is best, either Windows 2008 R2, VM Ware or Windows 2012 considering the cost factor as well?

If cost is a constraint, try Hyper-V. 
It's good but not as VM Ware. 
VM Ware is leader in virtualization and all its' products are tight integrated.
Also, plan according to you needs, whether you're going to Server virtualization or Desktop? 

Hi, In our company we use Windows because most of our product are Microsoft product.
Base on my own experience VM ware could be better for other platforms as Solaris, linux..etc.
TH coast is not the most important factor , you should also care about the Vendor, Resources about the product..etc   regards;  

You can use windows2008 R2 Hyber-V , its great and we have many production server installed on it .

ESXI is the best virtualization os for the following reasons.
Microsoft virtualization need three layers to get alive, 1-Hardware Layer 2-OS layer like2k8 or2k12 3-Virtulations    ESXI need only two layers  1-Hardware 2-Virtulations OS ESXI   So the Vmware Products are resource friendly.
you will not face idol time frequently and live migration and thin provision is working better on ESXI..

vmware

VMware ESXi is free, no licenses needed. Moreover, it's the state-of-the-art in virtualization. It occupies the magic quadrant at "Almost" all market research reports.

Cost wise and performance wise you are advised to use VMWare ESXi.

VMware is the kingpin of virtualization, but the game is changing fast and Microsoft is baking the technology into the very core of many products.
If you go with considering cost factor then Microsoft is the better .

without any second thought VMware  is the best option when considering the factors like

cost, performance and most importantly user friendly and most reliable.

VM ware or hyper-v the best virtual servers and the cost will be cheaper then the server