Generally I use a mix of both GUI and CLI applications and tools, for instance I am writing this using Nano within Konsole however I will start with what most people consider the important thing - how I interact with the internet.
I currently favour the excellent QuteBrowser which I switched to from Firefox + Vimperator, I also use Falkon as a secondary web browser (both are Web Engine based browser's), occasionally I Will fire up Brave however I am using this less and less, and use w3m or elinks (has gopher protocol support) more often.
I still use Thunderbird as my main desktop email application, I have used Mutt and (Al)pine in the past however due to the ease of set up of Thunderbird and Lightning's compatibility with CalDav based calender's its my go-to, If I find myself without X, I will usually use the mailx command to connect to my email accounts to read mail.
I use SyncThing across multiple devices to back up and sync various devices and folders. I used to use the SyncThing-GTK app however I now just use the webconsole.
I use KeePassXC on my desktop the database is synchronised and backed up using SyncThing.
Most people using KDE will use Dolphin, which I do - sometimes, I actually use MC Midnight Commander more than I use Dolphin. MC is an excellent way of managing files and folders, both local and remote. I also like ncdu, which is like a NCurses version of FileLight (KDE disk usage app)
While I have LibreOffice installed, I use Nano with enhanced syntax highlighting and spellchecking for most of my text editing and word processing needs (say no to vim and emacs)
Htop and sensors are classic ways to monitor your system, try glances as an alternative.
Imagemagick includes the convert command which I probably use more than most other image manipulation tools, although I do use GUI apps like Krita, GwenView and occasionally feh from the command line.
SoX (Sound eXchange) is a must have , what imagemagick/convert is to images SoX/play/rec is to Audio, easily play, edit, combine/merge, reverse and alter sound files from the command line, other command line playback tools include the 123 apps (mpg123,ogg123,flac123) gst-play-1.0 (gstreamer), I used to use CMUS but nolonger feel the need.
youtube-dl is an excellent way to convert youtube shows you like into mp3 podcasts while KDE's Dragon Player provides a simple GUI way to watch videos and streams.
I think this covers most of the regular apps I use, if you have a specific app that you use for a task I have not mentioned reach out to me on your preferred fediverse client @firstname.lastname@example.org and I will let you know what app I use for the same task.