"Imagine" there's huge economic downturn and you're company decides to cut everything (even free stuff...), what's the tool you'd fight for with your life?
Excel, always a way to perform quick and dirty data manipulation
I know a lot of people feel it's broken, and I'm interested to see Hey, the product, but I'm a big fan of being able to communicate in written and graphic form in short burst letters. Still incredibly impactful. I started Really Good Emails for that reason :P
Captio (http://captio.co/) - If your inbox is your to-do list or where you send things that you know you have to get done, Captio is invaluable. The "blank slate" simplicity of it makes quick notes or reminders or even long form on mobile incredibly easy. An app that I'd pay much more for.
Spark Email: consolidated inbox for both Gmail and outlook. keyboard shortcuts, it's fast. I did also previously use Superhuman, which would have been my answer, but they don't currently support Outlook.
Interesting thought experiment!
So since you mentioned free stuff, Google's apps—Gmail, Google Docs, and Google Sheets especially—would likely be the software that would be the hardest to work without. It's free for consumers, and so in my mind is free, but you're right, it is something that work pays for. Nothing else does collaboration as well, and Google Sheets with Google Forms is invaluable for building quick forms and getting data ready to use. At a minimum, an email service and an online spreadsheet in some form are worth their weight in gold.
Another free app I get way more value from than seems reasonable is the Preview app built into macOS. If Apple stopped bundling that, or if I switched back to Windows, that'd definitely be hard to give up.
It'd be hard to give up Notion now that we have so much stuff in it...
Something I've personally paid for that I'd have a hard time giving up would be a premium Markdown writing app—both iA Writer and Ulysses fit the bill here for me (and I pay for both). Sure, you could write Markdown text in any text editor, including Notepad on Windows or TextEdit on a Mac, but when you write a lot their design and formatting sure make writing a lot better experience. And, when you use a category of software much of the day, every day, that's worth paying for or fighting to keep around (which I guess now I understand why some people keep old PCs around or run virtual machines just to run their favorite version of WordPerfect).
It's either Excel/Google Sheets or Paperform. I'm constantly running numbers, cleaning lists, etc. in Google sheets. Paperform is also the lifeblood of our business. Think Typeform on steroids...we take payments with it, survey customers and even run customer surveys to create content.
Google spreadsheets and Gmail, but also Captio to take notes on the go, and 1Password of course :)
Slack would be mine. It can be a time sink, but in general it’s way more efficient than email.
Google Sheets. Excel with its lack of community integration is a nightmare!!
GSuite and Slack would probably be the last tools, way too much dependency on them, especially when remote
I recently added 1Password and Backblaze to my "home stack" to coordinate thing within my family. In addition, I use: 1Password - So my wife and I can share passwords Backblaze - Automated ackups...