Secure Platforms

Windows

No. Treat it like a game console but turn it off when you're not using it. Only use it for gaming. No banking, no conversations, no shopping.

Mac

macOS can be locked down pretty far but it removes most of what makes a Mac a Mac, the invasive and connected ecosystem. It's still an option though for Mac buyers with hardware that is not fully supported in Linux yet.

Extreme Privacy goes into it much better with step by step instructions but the gist is:

After wiping the machine, do not sign in to iCloud.
Install a software firewall (Lulu or Little Snitch) before connecting it to the network for the first time. Bring the installer on a thumb drive.
Turn off all invasive system features, including connectivity and syncing related features.
Be sure to encrypt the disk with FileVault.

Linux

The best desktop platform for privacy is a good Linux distro.

Good

Linux Mint or Pop OS.

Qubes OS is very useful but it is elaborate excess for someone needing to just leave

Avoid

Installation and operation has gotten easier, but if you're new to Linux you don't need to use Arch for basic privacy. The same goes for Fedora.

Kali Linux and Parrot OS are predominantly for professional work. They are not a daily driver for someone focused on privacy (even the one labeled as a daily driver).

iOS

Leave the iPhone ecosystem as soon as you can. Smartphones in general are a privacy nightmare. iPhones reach back to Apple regularly for a number of reasons. They even do it when you're connected to a VPN, sending that traffic outside of the VPN and informing Apple about where you are despite the VPN.

The operating system is closed source, has not been audited by third parties, and regularly sees outrageous security flaws. One example that's top of mind was the ability to remotely execute code by sending a text message or facetiming, even if the recipient doesn't pick up or interact with the message.

In recent years Apple has made great progress protecting the contents of locked phones. They even automatically reboot after 72 hours of being locked to reset them to Before First Unlock mode in case they are stolen.

Regardless, forensic software companies are able to unlock a number of iPhones even when reset to the Before First Unlock state. Being a popular platform also means they're a high priority for those forensics companies.

Rooting an iPhone does not improve the situation. While it may give you greater access over the processes running on the phone and allow you to wreak havoc among them in the search for more privacy, jailbreaking also fundamentally breaks the security of the phone since it depends on a software exploit to accomplish.

Stock Android phones

The versions of android shipped from mainstream vendors have the same kind of privacy concerns that iPhones do:

system services that report back to the manufacturer, around VPN connections
system services that report back to Google for functionality related to the Play Store, around VPN connections
shown to be unlockable with forensic software and popular enough research is always high priority for cracking the newest phones
likely unpatched vulnerabilities that remain to aid intelligence community backdoors

As mentioned above for iPhones, jailbreaking the stock operating system of an android phone would compromise the security and create a larger problem.

Degoogled phones

GrapheneOS, CalyxOS, LineageOS...