There is no "BEST" antivirus nothing is 100% effective, detection rates vary each day and AV tests are not so accurate
i use Norton Internet Security 2011 for my PC, and Avast Internet Security for my notebook.. there's no "the best" or "the worst", every AVs are the same.. they delete threats, don't they?? it's your decision to choose the right one for you, don't ask another person to choose the one...
I like Microsoft Security Essentials too, you'd think that nobody knows how to program a good a/v using all available techniques & undocumented APIs etc than the guys who wrote the OS in the first place. But it worries me how widespread its become, kinda makes it a big target for the bad guys.
Avast and Avira are equally good. But Avira porsonal has annoying advertising, and their update servers are slow sometimes. So my preference goes to Avast.
here is my experience.. for total security i recommend Kaspersky. when in XP days i like the part of kaspersky having "Self Defense", its against malware/rootkits disabling AV's, which Avira, Avast, Eset sucked at that. mostly all AV's are good nowadays since the day win7 came out. this is what i've done now. get a free AV. you can choose from (avast, avira, avg) then backed it up with Malwarebytes. (on demand scan only) whats on my machine? Sunbelt's Vipre AV. (Super low on resources and good detection rates.) Malwarebytes (on demand scan only) malwarebytes is good for cleaning things that AV's can't.
I also like Avast AntiVirus and MalwareBytes. I install these on many machines in my job. I have so many problems with Norton... just recently did a job where Internet Explorer would take ages to load pages. Removed Norton and problem solved.