Sign out of all Google accounts in your browser
Sign in to the Google account you want to use first
Sign in to any additional Google accounts after that
The first account you sign in to becomes the default account
On mobile, remove all Google accounts from the device
Add the preferred Google account first
Add any other Google accounts afterward
If needed, set the preferred account as the primary account in your device settings
In Chrome, create a separate profile for the default Google account
Use that Chrome profile whenever you want that account to stay default