If you are not going to be downloading movies (torrents), software or view movie sites that has unsecure web design, then the built in Windows defender in Windows 10 is enough. But if you do, any of the free antivirus is sufficient. If you want to buy, the best are the common ones like Kaspersky & Norton would be ideal.

The main difference between a free and paid version is down to features. Free will have file, some web protection and no firewall. Paid will have all those. But between them, the paid version would have what they call value added software like software update monitoring, windows cleanup tool and more. Some may be useful, some not.