>Microsoft Security Essentials
MSE is used as the baseline for all of the tests on av-comparatives, i.e. it offers the worst real-time protection of all of the tested antivirus products. In the august test Bitdefender and Avira come out on top. I have used avast in the past but I've switched to Bitdefender free now for a nag-free experience.
http://www.av-comparatives.org/dynamic-tests/
https://www.av-test.org/en/antivirus/home-windows/