Because of this forum, and also because of very knowledgable colleagues, I get more and more nervous about people using Keychain to store their passwords (it’s not really a disclaimer, but for what it’s worth I only switched to 1Password because my employer offers family use for free and I think I asked in here about people’s thoughts before moving). If one of your devices gets stolen and you get locked out of your Apple account, the chances of you getting back into that account ever seems to hover around nil. By extension, that would mean you lose access to every account you were using Keychain for, at least in the short-term until you can reinstate logins. The whole idea of this fills me with horror (I assume banks alone will take hours to sort, to say nothing of the fun and games you presumably get trying to verify your identity with new logins for Google, Microsoft, meta, etc.).
In contrast, if you’re using a third party password manager and one of your devices gets stolen and you lose access to your Apple account, that is still terrible but at least you still have access to all your logins and don’t have to worry about how you’re going to convince your bank’s website you’re the right person.
On to your actual question though, I’ve always been half and half with Apple apps.
Mail - I’ve never not used Mail (I want to switch, I just haven’t found something I’m comfortable with).
Notes - I have other apps, and dedicated uses for them, but Notes often floats around above it all out of habit.
Freeform - I love Freeform and it’s taken a lot of stuff that used to go in GoodNotes, but I really need Apple to implement folders soon. The lack of file management is becoming a source of friction for me and as I was scrolling trying to find a board this week I wondered if I was nearing my limit of it being useful until this function is implemented.
Pages - my workspace is Google, but I use Pages when I need to write up something that I’m converting to PDF.
Numbers - For personal use, I’ve used Numbers for over a decade and have some very old spreadsheets still in regular use. For work, only Excel will do.
Reminders - I keep lots of lists in there and always have done, but it doesn’t work for me as a task manager because I prefer to keep notes alongside my tasks and Reminders isn’t designed for that. That’s not a failing of Reminders, it’s just how my brain works.
My most used Apple app nowadays is Freeform, which is a bit odd when you think that it didn’t even exist a couple of years ago. I use it most days.