What are the most "cursed" American sports teams?

One thing that is truly great about American sports is the natural cycle of success.

10 years ago Who would've thought that the Lions and Bills woukd be some of the best teams in their NFL conference? Or that the Patriots would be one of the worst teams in the league?

It fits the American spirit that every team will eventually get a shot at "their year", but some teams just seem to get more unlucky every year?

Which teams are the most cursed?