Hi! Younusjavaid a warm welcome to you man! As far as i knew, the best OS for multitasking is windows 7 as the speed and performance of the system will never get down. So, I will prefer you the Windows 7 OS to perform the task.
I see we have another "what is the best..." thread that is probably going to generate endless discussion from the linux/Mac/Windows camps. Here is some enlightening and amusing reading for anyone who is tempted to respond to any "what is the best..." type threads.
It seems to me that most threads of this type are the internet equivalent of a little kid poking a hornet's nest with a stick and are generally not worth a response unless the question is specific as in "I have a specific task that I must accomplish with a limited budget and would like to know what applications I could look at to do this." Best is a term that should not generally be used unless you define the parameters. Best could be
best tech support
So save yourself the trouble of responding to open ended questions like this.