Windows Defender in Windows 8 is suitable enough assuming you apply just a little common sense, be careful with what you download and what websites you visit. Use it in combination with Windows UAC, Windows SmartScreen and Windows Firewall.
Don't go running unknown executables and make sure you only download from trusted websites such as Softpedia, Filehippo and Majorgeeks if possible.
I've been using Windows Defender in Windows 8 since the day it released and have not been infected, if I can do it right so can others.
Of course it doesn't hurt to have second opinion on-demand scanners like Malwarebytes & Hitman Pro for assurance.
Thanks.