That is a difficult Q to answer. It depends on user's preference. I use Kaspersky. You can try Microsoft Security Essential from Microsoft. Most people use "malwarebytes anti malware". This one removes fake antiviruses. . . . .
I also use Avast as a second line of defense, along with Microsoft Security Essentials and Trend Micro Titanium Internet Security. It is generally not advised to use two or more antivirus programs, but the new compatible setting of Avast allows the use of it without interference with other AV programs, and does not install the whole program. That option was unavailable with the old Avast version. It would disable MSE and Windows Defender. I disabled the Windows Defender service before installing MSE, however, and made each AV an exception of the others, so they would not scan each other. The Trend Micro discontinued their own firewall in favor of a booster for the Windows Firewall, which works very well. I use the Windows Firewall, but you could disable Trend Micro's firewall booster and install a compatible standalone firewall, such as Online Armor Free or Agnitum Outpost Free. I was using Online Armor, but I found that the Windows Firewall works just fine with the Trend Micro booster. Or, as some others have posted, you can use just one AV, if it says it is for Win7 64 bit.
G Data AV is a combination of Avast and BitDefender, and is also a good standalone program, as is Vipre, which has strong AV Protection Score for Win7, but not as strong for Win XP. Vipre and G Data will also disable Microsoft Security Essentials, so MSE should be uninstalled prior to using either of those programs. I can also say MSE is also a good standalone AV. Today MSE prevented a virus from infecting my computer, stopping it before Avast and Trend Micro detected it. Trend Micro does a fair job, although giving me a few false positives, which I have to instruct it to ignore next time.