There is no best operating system for Mathematica. In what sense would you want to compare operating systems? Mathematica should run fine on Windows/ OS X and most common varities of Linux. See this link for more information (
https://www.wolfram.com/mathematica/features/system-requirements.html).
There're many reasons you might be seeing Mathematica only use 20-30%. One common reason people see this is that they have a multicore machine and are running something that isn't a parallelizable process in Mathematica. In this case, the computation is restricted to one CPU. Another reason might be that the OS is allocating only that amount for Mathematica. This could be happening for a number of reasons, including that something else is running or there's another process with a higher priority. It's hard to say.