Windows Defender for me - works great with Windows 10, doesn’t get in the way and doesn’t cause problems with updates. Seems that most other 3rd party AV products these days, even if they offer a better level of protection, get in the way of Windows too much for my liking. I’ve been using Defender as my antivirus since Windows 8, and Security Essentials before that on 7 and I have had no issues!