There is no "best." It all depends on your needs.
There are some antivirus programs that lock down your system to the point that you need to approve every single step of your download. If you have less informed individuals mindlessly downloading files, perhaps these types of AV programs are right for you. If you are the type of person that can reasonably predict an unscrupulous file, these warnings will become tedious and this probably isn't the AV for you.
There are also some AV programs that work better with other software on your computer.
Everything mentioned so far is a reputable AV program. Since they are all free, test them out and see what you like. Not at the same time, because that causes it's own issues. But, download Kaspersky, Avast, and AVG and see which one you like best. Are the Avast warnings too much? Or do you like knowing when files are downloading to your computer? Can you easily Whitelist commonly accessed files in AVG to save time and resources? Or do you like Kaspersky or Windows Defender's layout a little bit more? Only you can answer these questions.
I think so. You can only use the free version for a short period. Watch out for special details in some of the computer magazines. I got two years Kaspersky on 3 PCs for around $50.
I like Avast and AVG. Both work quite well and have permanently free versions. Malwarebytes (free also) goes along with them. It blocks malware while the others block virii. Malware and a virus are different forms of attack software that need their own protection applications.
Free trials mean they are going to make you pay after a short period. (and often has something that gets installed that will not allow you to trash it and simply re-download it rather than pay!) Freeware stays free. (although they will try to convince you that you need some extra features that require the "premium" / PAID version. You really don't need the extra features.
No. Windows Defender running on Windows 10 is.
Personally I'm a fan of Avast.
From previous experience, I would have to say malwarebytes is the most effective.
no, AGV is.