I've been using BitDefender for a couple years... and Norton for many years before that.
I augment my security with Malaware.
I've got a lot of computers, and virtual machines as well... and from time to time I use some free stuff including Avast, AVG, and Microsoft Security Essentials.
I think MSE was good for a couple years, but can no longer keep up with current needs.
I think BitDefender does a good job but it is filled with bloatware -- lots of app to protect various elements of your digital security. It seems intrusive IMO but maybe that is needed in the current world with a variety of attacks.
From time to time, I am given computers from friends and family, and friends of family to disinfect. I have found that Malaware seems to do the best job of finding and fixing issues.
I tried Kaspersky once... i thought it was good, but also intrusive.
I think I'm ready to ditch BitDefender... but would like something fast, effective, and non-intrusive. (Still looking...)