Bottles, because it makes using Wine (for running Windows applications like Ableton Live) so much easier and not affect your system's Wine installation (because of the containerized approach). It's also FOSS and on Flathub for easy installation.

Each person has different needs, but assuming the newcomer to Linux has all of the basic applications/software preinstalled (browser, email client, office suite, etc.), and if you're using anything Debian or Ubuntu-based (since you're new to Linux), please, make sure to have Synaptic and gdebi installed. They will help you install software before you get the terminal figured out.

quickemu for quickly installing a Windows VM to use programs that don't run well or at all on Bottles.

#Firefox

AppImageLauncher because it doesn't require knowing any command to install appimage

LibreOffice because most people could benefit from having a full "office suite."

#keepassxc for your security

An IRC client (e.g. HexChat). You can usually get better tech support on IRC than you can by calling Microsoft or Apple tech support on Windows/MacOS.

This is theoretical and not necessarily an application (but it could be), but maybe a guide on how to use the Linux terminal (bash, fish, or zsh) and some common dos and don'ts.

"man"

keepassxc to save and retrieve passwords

GNU Coreutils

Timeshift, Snapper or Systemback - a snapshot application with short instruction how to use it correctly. Saves life if you screw something up.

fdupes

FreeTube!!! So the new user can watch YouTube videos without interruptions of ads. And not the other way around, like it is on YouTube. 😀

Flameshot. More DEs should have as easy to use screenshot tools by default, but many don't

#flatpak & #firefox

that's not easy there are several must have Apps for Beginner in Linux, mainly theese: Firefox, Libreoffice, Thunderbird.

just one? 😁
For what usage?

Firefox because it's the best browser that's protecting your privacy 😀