Opinion. Everyone has one, most argue against them. I've used Norton/Symantec for AV (and other things like Optimization for 25+ years, never had an issue, never had it fail me, yet some ppl can't stand it. Personally, I'm not a fan of AVG, I find it 'wishy-washy' in its abilities. So there's that.
Apart from Norton, probably the best AV I've found has been Kaspersky, it has a tendency to pick up on stuff that many AV don't. Just as I prefer to mix-up malwarebytes and Spybot: Search and Destroy.
Almost all AV/malware definitions/fixes that any AV type program uses are supplied by feedback from actual pc users, so if you do find a Trojan, it's only because someone else had it, heuristics found it, user sent it to malwarebytes, who made a fix and a definition for it. But, they might be the only ones to have a fix yet, and if you are using Windows defender and you get it, Defender will not see it. Nobody has yet sent it to Microsoft for review.
Actual virus's are pretty uncommon nowadays, too much of a pain to figure out how to get around in Windows code, and they don't pay. Trojans/malware are extremely common because they are non-destructive to the os/pc just your personal habits/info which is what many websites are after, so they'll pay ppl to write malware for their purposes.
Having AV is important, who, not so much, they are all pretty effective at catching the odd virus, but malware defense is extremely important. Is malwarebytes (free) enough? Imho, no it isn't, it's a very good checker, but there's always some disgruntled IT genius out there trying to make a buck and writing malware specifically designed not to be picked up by malwarebytes. Having 2x malware checkers is a better safety net.
Of course the best defense is a clean backup and the ability to wipe out everything and return your pc back to a clean state you had before the malware/virus got you in trouble.