Is Ubuntu the cheapest and best OS system?
It's free, so yes, it's pretty cheap (but almost every other Linux distro is also free). It's a pretty good place for someone new to Linux to start. There is a lot of information on the internet if you have problems in Ubuntu. It still has a lot to offer power users as well. (obviously a very simplistic description).
I currently use Ubuntu 14.04 for about 80% of all computer use (work desktop and my main desktop at home) - been working great for me. My laptop is windows.
Cheapest? No, because almost all if not all Linux distributions are free, not just Ubuntu. Best? No, but that's my personal opinion. There are many operating systems that are better in my opinion - including other Linux distributions.
Thanks, I am going to build my first PC after Christmas so should I use Ubuntu or windows without a key