At first Avast! had a few problems with Windows 8 but now everything has been sorted. I find it better than AVG as I tried that when Avast were sorting out teething problems.
Defender does a job but I'm not really happy with its performance.
For me Avast! free is my first choice, CNet link. When you download and start installing, check what other programs are offered and if you don't want them installed untick the boxes from the lower to the upper. You will have to register with your email address for a year's free licence.
Second choice is AVG free, scroll down for the free download link, again check when installing if you don't want another toolbar/program installed to untick any boxes. You'll need to register as above.
Lastly it's Defender I'd go with as your PC will run faster than with any other AV paid for or otherwise. Manually scanning the PC fully is abysmally slow, though, compared to the other two.
Safe browsing comes free :)