Knowledge is the best computer security, no substitute...
You can put 10 dead bolt locks on your door, however when you open your door and say "who is it", those locks don't mean much. Neither does security programs, same effect...
I repair computers often, typically from email, surfing, gaming and streaming. Not as much from porn as people think as they want you to return to their site, that's how they make money.
Some of the worst problems arise from games, especially free games, although even paid for games can have nasties. Kids are especially targeted due to the power they wield over their parents.
Social media games ALL have spyware and such. Anyone that says Facebook games doesn't have spyware are clueless at minimum. This applies to phones too as more phones are getting infected.
Streaming radio... Even 'honest' stations do NOT screen all of their advertising, same for movies. It is best to leave TV and music to dedicated devices that aren't affected by nasties. It is not if, but when...
Shopping assistants and tool bars are not your friend. Be very careful with downloads. Not merely the programs, but the sites that you get them from. Some very popular and large sites have been known to add goodies to the program without the developer even knowing. If possible, always download from the developers site directly...
Do not run more than 1 antivirus at the time. I'm not going to take the time to explain why, just don't...
Windows Defender was once good, however they didn't/don't keep up as they should and is typically disabled by decent antivirus programs upon installation and by many nasties as well.
Email is a whole vulnerability on it's own... Curiosity kills the cat...
Bitdefender Free is what I have been putting on machines for the last year or so. No such thing as the 'best antivirus'. (I am not implying that you should use it and have a false sense of security. Go back and read the first line in this post)...
It really depends how they rank 'consistently', how often they update and the 'quality' of their updates as many are not really functional on the loose in the wild...
I wouldn't use McAfee, Norton/Symantec or Kaspersky on any machine that I repair and always uninstall, although some corporate versions of Norton/Symantec are OK as system admins would not accept what home users are clueless about... That is totally different versions and much more expensive.
I must confess that I use a paid version of a different AV on mine really because I have to protect our 25 or so computers and the users from themselves doing silly things...
After one does get infected, it is usually best to run 4 or 5 different programs as no 1 can get everything. Malwarebytes is a good one to use for starters, as others have mentioned. SuperAntiSpyware, CCleaner, Glary, Process Explorer- Autoruns, RKill, TDSSKiller, ComboFix are a few other freebes, however you had better be careful and know what you are doing when using some of them. Don't blame me if you hose your box...
One more very important thing. Be sure of the spelling of sites and programs as bad guys often take advantage of carelessness in many ways.
Good luck! Be safe in cyber space!
Inquiring minds want to know...