I've run Avast and AVG in the past, but I've been running Microsoft Essentials for several years now, and it's all I need (mostly): it's reliable, never-intrusive, and IT WORKS. I also value that it updates as automatically and transparently as any tool I've ever used (since my 1983 Kaypro 10). Although it's all I need running live, I would never be without Malawarebytes for ad hoc and crisis purposes. I think Malawarebytes has the best crisis-support teams in the industry, besides having the best tools.
Disclaimer: I am among the least-paranoid users, AND I am not a browse-anywhere kind of guy.