MSE cause it's free and seems to be unintrusive, light and runs fine with Sonar on my old slow machine fine.
I've been through all kinds of antivirus and seems like MSE works best. Used to swear by zone alarm until upgrading became so problematic, resource hog and couldn't remove virus. Norton before that with same results as ZoneAlarm.
Bottom line, I think thier all crap. But at least MSE stays out of the way.
Agree with poster saying MSE isn't full proof.
Just be careful on the Internet.