My personal must-haves:
AdGuard for Mac. Ad blocking software is a must-have, but a lot of it out there is rather sketchy. AG4M isn't free, but I got a lifetime license while on sale and was well worth it. It covers all apps on your Mac, not just the web browsers.
IINA. It's a media player that will take anything thrown at it. It's faster and more efficient than VLC and is based on mpv, which is wildly popular today on Windows and Linux in particular, and for good reason.
Keka. It's an archive extraction and compression utility that excels at what it does. It replaced another popular extractor, The Unarchiver, because it handles certain rar archives flawlessly that TU chokes on.
AppCleaner. This app helps remove applications and most of the helper files that they create when used. There are several apps that do the same thing, but this one has proven to be more thorough for me than most.
Find Any File. Literally, you can find any file on your Mac using any of a variety of criteria. I've used it to root out app leftovers that even AppCleaner didn't catch, but it's useful for other needs.
Carbon Copy Cloner. It's an indispensable backup utility that literally can clone your internal drive to an external drive or disk image, then just update any files that have changed over time.
DetectX Swift. Malware is a reality on macOS, but dedicated antivirus software is overkill, especially since macOS does have built-in protections and screening. A second opinion never hurts and this app can watch the different places where apps are set to launch on login so you can get a heads-up when they are modified, whether legitimately or not.
Onyx. Already mentioned, but a good utility to run if you are having issues with your system. I usually run it after a macOS update to rebuild the cache in particular.
TinkerTool. This app can let you modify "hidden" features of macOS. It's free and very handy. It has a sister application, TinkerTool System, that isn't free and while it replicates a lot of the features of Onyx, it has a few other nifty tricks up its sleeve. Not necessarily "essential", but I mention it so you don't confuse the two TinkerTools.
Xee³. It's a graphics viewer that is IMO the best available for macOS. It's not free, but not crazy expensive either.
1Password. It's for storing and managing passwords for websites, as well as other things like software license codes. The reality today is that you cannot re-use the same password on multiple sites on the internet because if you do and one site gets compromised, then you are compromised everywhere. There are other options available, and while I have long used and prefer 1Password, their pricing is getting to be a bit crazy by pushing people to subscriptions, something you can avoid if you carefully review all their purchase options.
Cookie 6. Not essential for "everyone", but if you value your privacy online, this app will help you manage the cookies that websites store to minimize how much they track you.
Brave Browser. Some sites just don't work in Safari quite right so you need an alternative. I'm very happy with Brave, which is based on Chrome but more privacy minded. Firefox is a good alternative also.
I have lots of other essentials for my usage, but we'd need to know more about what you plan to use your Mac for before recommending other apps.