I've had some issues with Avast before so I switched to Kaspersky, no issues with them so far. I also set exclusions so when I run resource intensive programs or games Kaspersky turns off it's shields until the application is closed.
My vote goes to Avast.
Its good detection plus a lot of setting for advanced user. And it has clean interface as well.
KFA also a good choice, it's just that I like Avast more in this case.
Avira detection is good. But its GUI is a totally joke. One part has new design, other part still using the old one. Removal of very useful feature in that past that is the ability to read log file easily. Also I hate the useless launcher.
I don´t like Avira free, it´s not comfortable.
Avast is very good with a great BB, but I only install its 3 first options (File+Behaour+Web). I don´t like about Avast that it always slows my boot time.
And yes, Bitdefender free deserves to be on this list.
In the end it all depends on your knowledge and your computer. but my list, in order, would be: Kaspersky, Avast and Bitdefender, and and combine them with AppcheckAntiramsomware if I don't want to click the mouse or one of the following list: NVT ERP, VoodooShield, SecureAplus or Comodo Firewall if I don't mind interacting with the computer
Whatever you choose between Avast, kaspersky or Bitdefender will be a good choice, excellent if you combine it with the list above
My top free AVs are either Kaspersky Free or Bitdefender Free. Kaspersky Free + NVT OS Armor is a strong, proven stable configuration from my use. Other AVs like Avast, AVG or Avira primary function is to display ads, track its users usage habits etc for revenue. Anti virus function is just a complement feature to them for using their software.