For me the order goes Avira Antivirus, AVG, Microsoft Security Essentials, Avast, all the rest (which are the ones you shouldn't use). AVG was always my preferred anti-virus from 1998 to about a year ago. Then I decided to try Avira and I feel it has a better positive detection rate than AVG. AVG does signal some false positives as already pointed out and sometimes it misses things. MSE is actually really good but I prefer Avira due to familiarity. Avast is good as well but I find the UI hard to work with and too graphical for it's own good.
My recommendation: Avira Antivirus (http://download.cnet.com/Avira-AntiVir-Personal-Free-Antivirus/3000-2239_4-10322935.html)







