Windows Defender is a resource hog and falls short compared to the others.
MBAM Free doesn't have real-time protection, not to mention it's a greater resource hog than Windows Defender.
Avast Free has the privacy concerns not to mention the constant compatibility issues with Windows 10, but beyond that would be best option from all the mentioned.
BitDefender Free lacks customization, is solid in terms of protection, but known for being buggy.