IMO there really is no best answer for this. There is a combination of things that can keep you from being infected though. First and foremost is being careful: don't download from sites/sources you don't trust, don't go to the bad places of the internet, when you are installing a program make sure to READ each section of the setup instead of just clicking next (I have been totally guilty of this in the past and got unwanted installs as a result), and DON'T click any popups (always close the window). Second, a decent free AV will work fine as long as you stick to rule #1. I use Avast free, MSE is actually quite good and really low resource use, and there are several other free options out there that work great. A quick Google search for AV software reviews can bring up ratings and rankings that you can go through and find out what is best for you. And third, supplement your AV suite with Malwarebytes. Setup a daily or weekly Malware scan to supplement your AV because Malware is usually what free AV software is weak on.
Obviously this is all my opinion, but this has worked for me for quite a while 🙂 Hope this helps.