I have three that I couldn’t imagine my computer use without. Here they are, in the order of importance:
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Kate, my favourite text editor. It’s just amazing. What I love about it: LSP support for C, C++, and LATEX, customisable syntax highlighting, block select mode, multiple cursors, embedded Konsole, advanced formatting shortcuts (like automatic reflowing of lines, adding comments, swapping and duplicating lines), auto completion, split view… I do all my writing and programming in it. I find it much better than VSCode (which Micro$oft poured millions of dollars into). I have to admit, at this point, I’m completely addicted to Kate, losing it would set me back years (I would have to go back to Neovim, which is great, but it’s not Kate).
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Konsole, it is the best terminal emulator in existence. It has all the fancy features I would ever want from a terminal emulator: split views, tabs, powerful scripting options, truecolour and unicode support, huge scrollback, clickable links and files. And when you run it without borders and with transparency turned on, it looks very nice, too. I do all my file managing and program launching tasks in Konsole. Zsh is my shell of choice.
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Okular, a great document viewer. Very fast, stays out of the way and displays the document. Can exchange light and dark colours and tint everything, for viewing at night. It can automatically reload documents (very important when writing a paper in LATEX - Kate on one monitor, with embedded Konsole, Okular on another, very comfy). The only feature that I find missing is very fast, middle mouse button autoscroll for quickly skimming through books.
In fact, those three applications are the reason I use Plasma. Plasma is nice and everything, but if it didn’t have those programs, I’d probably just use dwm. But since I have to pull a few gigabytes of KDE dependencies with Kate, Konsole, and Okular, I may as well just use Plasma. 
Other great KDE applications:
Dolphin is also pretty nice, especially the embedded Konsole, and how it automatically cds into directories. In fact, for me, Dolphin is just a visual extension to Konsole.
Gwenview is great, and so is Krita. I also like Kdenlive. Filelight is very pretty. LabPlot, too.
I’m not sure if this is a true KDE application, but I really love Frescobaldi. It is the only “text editor” I ever use beside Kate. I wish Kate had some LSP support for Lilypond, but Frescobaldi saves the day in that regard.
What I find missing from KDE applications: a good audio editor, on par with Audacity in terms of features, and a decent, small, fast, and simple DAW.