Update Nov 2016
I haven’t used any 3rd party Anti-virus since upgrading to Windows 8.0/8.1, with the exceprtion of Malwarebytes pro (now home premium.) I’ve used Malwarebytes (MBAM) for many years as part of my “virus/malware removal tool kit and I’ve found that MBAM removes stuff that other (paid for) AV products fail to remove.
Windows Defender, imo, does a great job and plays nicely with MBAM. The products basically do the same job, in a way, but do so differently, which means they don’t get in each others way. It seems to me that other 3rd party AV products actively disable any protection that Microsoft have put in place to enable their products to work but that might not be the case with all 3rd party AV products.
Microsoft have now taken charge of security updates and have made Windows Defender a worthy AV, couple this with the new improved software firewall and I think you don’t need 3rd party AV but would always recommend MBAM as an addition to Windows Defender.
I have used the following products and can hopefully give you a non biased opinion. I have used others not mentioned here but the ones below are my “pick of the pops”
Type: Internet security suite (These products include all the above and a firewall)