Windows 8.1 already comes with the Windows Defender as a built-in antivirus/antimalware solutions. If you are satisfied with it, then you can keep it. There is no such thing as the best antivirus for a certain operating system. You will need to try and see which one works the best for you. For example, you can browse the antivirus solutions from the database of Software Informer. Read reviews and guide yourself based on the ratings, and then you can pick one to install.
Antivirus results: http://software.informer.com/search/antivirus
