Windows 7 built in Defender is old and obsolete. You are not safe using it.
Any newer anti virus installed should turn off Defender.
What is the best protection is up to the owner/operator of the computer.
I use Microsoft Security Essentials and Malwarebytes Anti Malware Pro; both run active. I use Windows 7 built in firewall.
Microsoft Security Essentials - Microsoft Windows
Malwarebytes Anti-Malware - Protect, Detect & Remove Malware From Your PC
I also have a few on demand programs I use when need be.
Their is no one protection programs that does it all.