Linux

Show her some pictures or videos of DEs and see what she likes. If she's someone who likes to make it look the way she wants, she might get a bit more out of KDE than Cinnamon for example.

I would recommend either Debian or Devuan - both are absolutely rock stable and are a good entry level drug for the Debian based ecosystem. I personally like Devuan more (it just feels more mature and has more ~~old~~ mature community members).

You could install bunch of popular liveboot distros on USB with ventoy and have them try each one. Just make sure to mention it will run faster when not in a USB.

Did this with my SO, they have mint like me. And they like it!

They wanted puppy linux though xD

I'm honestly astounded at how many people are suggesting Mint. I recently switched full time to linux and even as a software dev, Mint has to be one of the worst experiences I've had with a computer. Not only driver issues, but software issues and general buginess. Along with being butt-ugly, I do not think any windows user is going to confuse Mint for Windows.

I switched my wife to Bazzite (not necessarily recommending that) and she literally didn't notice it was a different operating system (even though I told her it was and walked her through it). Bazzite has a nice UI for installing pretty much anything a normie would be thinking to install. The only issue we've had so far is that Dropbox just outright does not work on it. I've filed a bug with them and have been awaiting a response from their dev team for like two months now. I'm sure they'll fix it eventually, but if you need the Dropbox UI (you can use rsync otherwise) then don't choose Bazzite.

As for myself, after trying out like 6 different OSes, I settled on CachyOS. There are still issues, but it's pretty dang stable and they're very fast to fix issues. It's not for a person not willing to touch a terminal at least once though.

Coming up on 10 years since I switched from windows to Linux. I tried Ubuntu and absolutely hated it, so much so that I switched back to windows at first. But I kept reading and tried ZorinOS, and that got me comfortable with Linux, it was a little buggy but I could understand it.

After a few months with ZorinOS I switched to Linux Mint and have been running Mint for 9 years. Recently my 76 year old mother who has trouble with some basic computer stuff said she'd like to try Linux and asked me to help her, I made a live USB of Mint for her to try and she told me "I can understand this, it's like windows 7!". If she can get Mint, I feel totally confident recommending it to new users.

Everyone hypes Mint but if you're working with newish hardware you might have a bad time due to the drivers taking a while to mature and filter down through all the distros. If her rig is a couple years old it should work just fine though. I would also suggest trying out Kubuntu, Pop!_OS, PikaOS, and Zorin if that is the case.

If she is on brand new hardware then something Arch based is the way to go IMO. CachyOS, Garuda, and EndeavorOS are all Arch based distros that make setup easy and they've all worked great for me out of the box. Honestly if you have snapshots configured with timeshift or something being on a rolling distro isn't as scary as it's made out to be. Fedora is an option too as they get updates every 6 months, but there is a little extra setup to do after install like media codecs and proprietary drivers etc.

Cachyos was my personal pick and it's working perfect for me so far.

Personally, I don't think anyone new to Linux at this point, who isn't tech-minded, should be pointed to an X11 environment. So until Mint devs have ported Muffin into a Wayland compositor, I wouldn't recommend it. They're used to a shiny experience visually, so I'd go with Plasma 6 running on Fedora or OpenSUSE Tumbleweed.

Put Linux on one of those spare hard drives and simply mount the existing drive as a second drive in Linux.

This will give you access to all your current files from within Linux without having to do anything. Move over what you want and need as you use Linux. At some point, you will probably want to reformat the original Windows drive for extra space. You could consider mounting it as /home at that point.

Choosing a distro is a matter of taste. I can tell you though that I have moved a few Windows users to Linux Mint and they are all happy with it. My last one was LMDE (Mint with a Debian base).

Distro:

• First choice: Mint Cinnamon
• If the GPU is very shitty: Elementary OS (Mint Cinnamon expects a basic level of GPU performance)
• If Mint/Elementary are too simple: Fedora KDE

Process:

• For fully switching: Obtain an external hard drive, copy the contents of the Windows partition(s) to it and install your preferred distro so that it takes over the entire computer. This is the most stable way.
• For dual booting: Buy an SSD for Linux, disconnect the Windows drive and install your distro of choice so that it takes up the entire space. Reconnect the Windows drive afterwards and set boot priorities in UEFI.

One More Tip: Don't frontload them with information, but teach them one thing: How search for and install packages through the GUI (Mint Software Manager/Elementary Store/KDE Discover). Tell them that it's more like a smartphone apps and downloading software from websites should be a last resort.

Debian or Ubuntu because they're stable and well-funded. Makes a lot of stuff easier.

Bazzite

Mint. it's slick, stable and similar (usability wise) to people coming from windows

Fedora. I would not have said that two years, but I am blown away by how easy and up to date it is.

And I am normally an Arch person.

This really depends on her hardware specs and what applications she needs to use.

Without knowing any of that, I would suggest Linux Mint. It is desktop user focussed and a good general OS. It includes drivers and common software in their version of an app store.

Debian is my distro of choice, but is not ideal for a new Linux user.

I would suggest checking what apps she needs and making sure they are available on Linux, or that a close equivalent is. Any apps that will be replaced, try the replacement out on Windows first if available. For example Adobe Illustrator to Inkscape, or MS Office to Libre Office.

For data transfer:

1. As others have said. Backup the current computer fully. This in probably best done on an external hard drive. Make sure you know how to reinstall windows and restore from the backup.

2. Copy all her data onto a different external hard drive. This is not the backup. It is a separate drive.

3. Make sure all the data is actually on the external hard drive and readable from a different machine. Ideally boot from a Lunx live USB and check that the data can be accessed from the external drive.

4. Install her distro of choice.

5. Copy her data from the external HDD to her user account's home folder of newly installed Linux.

I vastly prefer/recommend stable LTS distros. There are really 2 main families of distros for this:

1. Linux Mint / Ubuntu LTS / Debian Stable (Ubuntu is based on Debian, Mint is based on Ubuntu LTS):

Basically endless amount of packages. Most people in the linux world have some familiarity with these so it shouldn't be hard to get help if you need it.

2. Rocky linux / Almalinux / RHEL (Rocky and Alma aim to be compatible with RHEL software):

For desktop systems people usually opt for fedora, but that distro does not meet my own criteria. Biggest reason you'd use these is for professional VFX software support. For whatever reason a lot of that stuff only has official support for this family of distros. Not sure why!

Get good at 1 of these families of distros. If you aren't vibing with one its okay to switch to the other. Both have more cutting edge options if you desire them.

Linux Mint is a community favorite and very much is built with a desktop user in mind, but I don't think it's unreasonable to subject someone to learning any of the others even if they are more server focused. Everything I listed has atleast 5 years of support! If your fiancee isn't super tech literate, you'll probably be the one doing a lot of the system maintenance so keeping those major updates sparse is a very good thing. And of course, if you don't wanna learn 2 different sets of tools, try and keep in the same family of distros.

Also, for desktop environment don't choose anything crazy obscure. KDE & Gnome are most common, Cinnamon & XFCE are less common but IMO fine. Venture into others at your own peril.

Transfer process depends on what you mean. Transferring your files will probably just take time. I'm hopelessly unorganized so for me backing stuff up takes a few days of combing through a bunch of junk and copying to a flashdrive or cloud storage. Other people might have more efficient ways of dealing with this though.

If you mean software Libreoffice is great local office software, SMplayer is imo a good media player, GIMP, Inkscape, and Krita got art stuff covered. We're also at the point you can more or less run most windows software on linux with enough fiddling, but that obviously isn't ideal.

Your biggest hurdle moving to linux full time will be understanding commands when you inevitably do need to change configuration of something with the terminal. If you need help there are usually forums, IRC, matrix, etc.

Happy computing!

While I'm here, I might as well figure out one for me, I usually stick to gaming and graphic design programs since I'm an artist. but honestly I do anything under the Sun and whatever my whims fancy so flexibility is a must