Kwrite. I write a lot of text at work and I hate using word processors. I use this app for its excellent autocomplete feature. You type a word once and for next instances you can use tab to auto-complete. Very convenient.
I work mostly with Vim but when some task is easier with a GUI, Kwrite is the best. It's so quick, even with a lot of text. And that preview scroll bar they added recently, 👌