In most cases the cracker doesn't care which password he cracks, as long as he gets one of them, so the weakest password will let him into the machine. The best password is a mixture of letters, numbers, punctuation and special characters. The more complex and random it is the harder it will be for someone else to crack. Of course it may also be hard for you to remember, so you should try to choose a complicated password which is also relatively easy for you to remember, but hard for someone else to guess. Use at least 6 characters in the password.
Shorter passwords are easier for computer programs to guess.
Newer versions of Unix require that you use at least 6 characters in a password, and that at least one of them not be a letter. But remember that most Unix passwords are only 8 characters long - any extra characters are simply ignored. Just adding a number or punctuation mark to a word can make a password a bit more secure, but if it's a dictionary word then this will probably not enough. Use fragments of words mixed in unusual ways that would not be found in a dictionary, or take a compound word and swap the pieces in an unusual way. The password just suggested above is even better if you swap the first and last parts.