Honestly, its gotta be the MS Office suite.

Yes if you’re just writing your own simple documents libreoffice/OpenOffice will work, but if you have to do anything more complex than a single page spreadsheet, text-on-white presentations, or 3 page MLA book reports… or, even worse, have to interact with documents and spreadsheets created by basically any other person on the planet, I’ve just never had a good consistent experience with any of the free options.