I think the best way to select a strong password is to let Google Chrome or Safari choose it for you.
They have "suggested password" option whenever you want to create an account.
They create a crazy complicated password and suggest it to you, and you just have to say yes.
Then they save it for you on your Gmail. You just have to make sure your Gmail account password is strong and turn on the extra security feature for it to make sure you don't loose it.