Microsoft Security Essentials / Windows Defender has been falling behind for years now. It used to be pretty good. But now, it unfortunately doesn't catch a lot newer malware. Microsoft dropped the ball and stopped putting the proper R&D into their product.
Bitdefender Free is my new favorite these days:
Fast, effective, and low impact. Bitdefender Free is not free for commercial use, however. And they don't have a free version that support Windows 10 yet. Bitdefender scores at or near the top in most AV comparison tests for malware detection.
Best free commercial AV is Avast for Business:
Not quite as low-impact as Bitdefender, and not quite as effective, but it's OK. I've used Avast for years as well. (It used to be my standard free AV, and I still use it on some systems.) Their free business AV is basically their paid AV business product stripped down to just AV, not firewalls, and anti-spam, and other cruft. The Windows firewall is just fine these days, and is you have a decent mail server spam isn't a problem. (And there are other decent free anti-spam products, like Cloudmark Desktop One.) So a plain-old just-AV product is fine with me. Includes a cloud-based console system as well, so you can centrally keep track of your AV clients -- which is GREAT for a free product.