TeX -> PDF ... but I'm looking forward to trying this next time:
Google Slides has come a long way since it launched. Has much of the feature parity with PowerPoint, better real-time collaboration and is free with a Gmail account.
emacs with org-mode and org-reveal. write a plain text file... get s presentation. use plant uml in the same text file and get a presentation with diagrams as fast as you can think.
Powerpoint. I have contempt for Word, like Excel for editing tables (if it doesn't matter if it gets math right)
But with Powerpoint I can take "boxes and lines" diagrams right out of my head and onto the screen. It's also pretty good for making print graphics on odd-sized cards, say a 5x7 card that is used to make a "holder" for a punched card decoder, or a 8.5x14 poster about recyclable vs. not-recyclable plastics.