This is of course subjective. But on all the computers that I have been fixing, cleaning up, or re-installing windows on for friends (6 in the last 8 weeks) I have been installing MS Security Essentials. In general I have been really impressed. The integration with the OS, it's ability to catch issues, updating, scanning have all been more than adequate in what I have seen so far. Everyone has been pleased with the results. I'm not sure there is a "Best" in every single category, especially for free, but so far I really like it. And to be honest, I really don't like much from Microsoft and generally use Linux for our family personal computing.
I generally use Firefox, have never had problems with Chrome, it does seem faster, my kids love it and use it primarily.