Make a password you could never have guessed.
Random characters for a password manager to remember, or a passphrase of random words for the few you must remember yourself.
The strongest password is one nobody chose. This generator draws random characters, or random words for a passphrase, from your browser's cryptographic random source (crypto.getRandomValues), then shows the entropy in bits and a rough offline time-to-crack, so you can see exactly what each extra character or word buys you. Use random characters for anything a password manager stores, and a passphrase for the manager itself and your computer login.
Humans are bad at random. Asked to invent a password, we produce a word, a capital letter, a year and an exclamation mark, and cracking tools know that. A generated password has no pattern to exploit; its strength is simply the number of equally likely possibilities, shown here as entropy in bits. Every extra bit doubles the attacker's work.
Passphrases work the same way: five words chosen at random from a list of a few hundred is not "five dictionary words", it is one of hundreds of billions of equally likely combinations, and far easier to type than symbol soup. Use random characters for anything a manager stores; use a passphrase for the manager itself and your computer login.