I have a few different passwords, one for email, one for games, one for forums, one for social networking, two for my PC user and admin passwords and one for sites that know my bank details. Each has a different level of strength depending on the cost to me if it was cracked.
The best are random streams of letters (upper and lower case), numbers and punctuation. They are difficult to remember so I have a system for devising them:
1. Start with a made-up phrase or nonsence word that means something to you and noone else.
1a. If it's a phrase, use only the first letter of each word to make an acronym.
2. Transliterate letters into numbers that they resemble: B->8, E->3, g->9, I->1, O->0, S->5, Z->2.