It really depends on what you're cooking and how you like your BBQ. I've not had SM, but have heard good things. Plowboys is great stuff, but knowing which one is right for you is dependent on if you're cooking pork or brisket or chicken or ribs. If you like it sweet or hot, etc.
Check out the basic rubs from the Chris Lilly book. Lots of good basic rubs to start with there.
When I die, my goal is to have a visible smoke ring when the coroner cuts me open for the autopsy.