You could say that for one year, the team with the worst record is the worst team... but that's dependant on so many factors. Bad management, a tough schedule, injuries and the like can turn a good team into an underperforming team.
If I had to pick the "worst" team, I'd have to say it's Pittsburg. They have tradition, following, and a good sized market, but they cosntantly sell off their young talent, and seemingly get nothing in return. In a city that is currently defending two championships, there's no reason that the Pirates should have 16 years of losing seasons.
DC and KC have money and attendance issues, and Baltimore plays in a truly brutal division, Oakland has had success within the past 10 years, Cincinatti was supposed to be a contender in the Central, but were plagued by injuries... these are contenders but their circumstances mitigate their poor performances.