Nice post to stir some shit, OP. 🍿

MacOS.

I hate Apple, and I do not like how they operate. But I cannot deny how user friendly their OS is, how affordable their machines are (Mac mini) and how even those who do not know how to use a PC, can pick them up and use them.

Linux Mint is my second choice

Only Linux which feels like a normal PC, and 99% of features can be installed via UI. There are holes, and I feel an immutable OS would fill this niche better, but for now this is my number 2.

Idk if I'd even recommend anything anymore. Microsoft shit is easier but more likely to just nuke itself or destroy your data. MacOS is stable and user friendly but prohibitively expensive. Linux is generally great, but requires a level of teach savvy that most people are actively afraid of. I'm so tired.

Linux, I'd say Manjaro, Linux Mint or Bazzite

MacOS (less with each release). One of the Linux distros that aims to minimize the "Linux" experience for new users, like Mint. There are others that are also good for new users who don't care what's underneath. I'll let others suggest them as I don't keep track of them all.

Linux. Hands down. Always.

New user? Try Kubuntu Linux

Power user? Eh, you can try anything but I'm still with Kubuntu because Ubuntu with KDE just works so damned nice

Are they looking to become savvy? Linux. Are they looking to game? Steamdeck, or if they can wait a while, Steam Machine. Are they just wanting a device to check emails and watch *ahem* 'internet videos?' Android phone.

Any other option will be more money for less value.

Whatever Linux is being sold pre-installed on a machine within their budget.

Told my dad to buy a Dell laptop with Ubuntu preinstalled after his last Windows laptop died. He's been fine with that for the last 5 years.

Linux - if it's on hardware that is sold specifically as fully compatible (like a Framework laptop or something like this)

Debian Linux

I have installed it for several computer illiterate old ladies. They swear by it.

Apple for sure. I dislike them personally, but their OS has a great UI/UX and is very user friendly. And their stuff just works together really seamlessly.

But, if all they do is browse the Internet, Linux is also a good option as long as you're ready to set it up for them and answer questions. I switched my 75yo mom to Mint when Win10 hit EoL and she hasn't had an issue.

What do they want to do?

An iPad with a keyboard can serve the needs of a significant amount of users. I know people for whom it’s their only device apart from a phone.

A hardcore gamer will want Windows. Linux can play a lot better of games natively or fairly easily via Proton. macOS has some good ones natively but playing via Wine is more complicated. Some specialized industry workflows will require it. Windows-only games and applications are the only reason to recommend Windows.

Mac/macOS is great general purpose hardware and software. Good applications are available for art, science, engineering and productivity and it’s certified Unix. The ecosystem can be slick—Handoff from/to iPhone, unlock with Watch, TimeMachine backups, etc. Support people are usually native speakers from the same country. Some good native games and some through Wine are possible but Window and Linux have significantly more.

Linux can work for basic email, productivity and web browsing. Gaming choices are better than macOS but Windows is still better. They don’t have to worry about ads or the next interface fad being forced on them. They’ll likely need a bit more hand-holding and support down the road. They’ll need a Linux guy but they can be up and running on old/cheap hardware.

MacOS combines user friendliness, easy learning curve, seamless integration with other devices, and reliability.

It’s what I’d recommended to anyone who wants to use a computer without having to deal with the aggravation of dealing with a computer.

I'm a bit at a loss here with all these comments - can someone (for the sake of argument(s) & me giving further advice on such topics) give me some plastic examples what kind of issues bother average users of Windows, Linux, and macOS?

Like, is this like with rare(ish) cases, eg car accidents, where users don't usually experience them regularly so it's hard to argue which is best for which situation bcs every case is unique?

Bcs in the past couple of Windows iterations & "main" Linux distros (let's say last 15 years?), afaik the average user doesn't really have any questions on "how to use the system", nor encounters breaking changes overnight.
Even more to the point, an average user will need tech support regardless of OS if any kind of problem occurs (and in most cases "a restart" will fix it anyway).

To my limited Windows exposure in the very last few years I would say the most common issue I got asked about was either "why did this new thing appear now" or a stuck Windows update (either from buggy colossal space gobbling (as a bug of a current update or collected temp files that never got cleaned), or unknown reasons as per MS statements).
(I have literally 0 W11 experience, the above were Window 10 issues.)

To my limited Linux exposure in the very last few years the only actual question I got to actually research to solve was a version stuck nVidia driver (multiple repo issue which I guess distro upgrade didn't handle properly), which isn't dissimilar to issues Windows had with graphic drivers decade(s?) ago (when the previous version didn't fully unusual & it caused issues, sometimes even Windows update would bork a manual driver install, which is kinda like multiple repo issue).

(I also tech support a few family & friends with their techy needs, including PCs, but none of them are demanding users. They all "got switched" :) to Linux from Win XPs, Vista, and 7s, and since through various distros over the years, mostly Debian, are currently now like 5 years on Tumbleweed & the only question I remember in the last years, which I got multiple times, is why doesn't the daily wallpaper tell them where the pic is from bcs they remember that at one point "it did" - idk, some Gnome default thing did that iirc. I got the same question from by coworker about her Windows daily wallpaper, bcs it doesn't always display the info in lock screen.
Oh, and from my overall tech support to family most of the time is spent on passwords they have forgotten and ISPs dropping a connection for like 2 seconds once a year, bcs boomers.)

iOS.

Linux only if they can follow directions.

Average user needs their computer mainly as a device to run a web browser. Mac does that the most reliably. This seems objectively true to me.

If they're a nerd, then either Linux or Windows depending on what kind of nerd they are.

Windows for gaming.

Point blank generic recommendation? MacOS.

Otherwise, Linux is the endgame, so it's a matter of talking to the person to see what software is essential for them.

Linux Mint

Going to have to go with TOS v1.64

Linux mint

If they are not savvy enough to do troubleshooting themselves or have no one to ask for help, I think macOS would be the best (assuming money isn’t an issue). Otherwise, Linux Mint.

TempleOS

MacOS if they can afford it. Otherwise Linux Mint.

As someone who has used all three (I main my Mac for school, Linux for my gaming PC, and windows for VR gaming)

The answer is either windows or Mac.

Mac if they can afford it, windows if they just want to use it.

Nearing my 60s, very satisfied Linux Mint user. Obviously, it all depends what the user expects from their computer.

I like Zorin Linux.

I would not wish Window on my worst enemies.

Depends on what they need it for.

Gaming, rendering, anything that needs lots of power and driver support - Windows

Music production, video editing, graphic design - MacOS

Programming, or people who want full control over their system - Linux

Don't listen to Apple fanboys. You absolutely do not need to spend $1000 to take notes and browse the internet.

Mac or Linux, something like Mint, Zorin, or PopOS.

Mac for the better ingregration with more popular apps. Other than that, Linux, but test it out first since not all hardware works correctly. I had good luck with hardware on all that I have tried so far.