The smartest move when it comes to passwords on your smartphone is to use a password manager such as 1Password. A password manager can not only create a super-secure password, it can remember it and use it to log into your accounts for you, making the whole password thing both safe and simple.
If you're still coming up with your own passwords; however, then there's a good chance that you forget them from time to time. If you're trying to log into a site that you've saved the password for in your iCloud Keychain (that thing that makes it so the password field autofills), then you can actually ask Siri what the password is.
CNET pointed out this week that if you say "Siri, Show me my passwords" it will launch the passwords page in settings and show you all the passwords you have saved. Alternatively, you can just ask Siri for the password for a particular site or app and she'll take you to the account page in settings with the password information you requested.
Both options require you to prove your identity through Face or Touch ID, so random people who have your phone can't use get access to all your passwords.
And if you don't want to ask Siri, you can always just look the info up yourself. You can find your saved passwords by going to Settings, followed by Password & Accounts, and then Website and App Passwords.