For most people who have good internet browsing habits, and don't install a bunch of random software I would say Defender and Malwarebytes Full Version (yes it's worth purchasing, the full version gives you realtime protection).
However if you are a user who installs lots of random software, downloads lots of files (especially from peer to peer file transfer programs), visits porn sites, etc you may want a full fledged Antivirus program (I personally recommend ESET NOD32).
That is my opinion, it varies person to person completely depends on your internet habits. Remember no antivirus program is 100%, new viruses come out all the time and if they are not in your antivirus programs definitions (or dictionary if you will) they won't be able to catch it. So even with an antivirus program you can still get viruses, but if you use a good one with a great reputation and detection rate (such as the one I recommended) you will be more protected.