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.
In my opinion, a light AV is a better option, something like malwarebytes. The point of viruses is to bypass anti-viruses. Getting an AV like norton or similar AV's that hog CPU usage is a bad idea. The thing that protects you the most is common sense! If something seems too good to be true, don't download anything. This is just my opinion though. Its also from past experience, when I was experimenting with malware and trojans. You'd be surprised how easy it is to get past AV's, even the most stubborn ones like norton. Hope this helped!
This question will open up a can of worms here , everyone's opinions differ as to what they believe is the best antivirus.
I suggest you check out any antivirus you are looking at and look for reviews etc.
If still undecided all antivirus progs will give you a trial , download a trial , see for yourself if it suits your needs , if not , uninstall that one and try another.
I personally use AVG Free Antivirus it uses little cpu usage and has an extensive antivirus data base , worth a look
I wish you luck , dont just rush out and buy , try .
also there are lots of free versions out there also.
Hope this helps