5. International Brotherhood of Teamsters
The Teamsters might be the best-known labor union in the United States because of its alleged ties to the mafia and the famous disappearance of one of its former leaders, Jimmy Hoffa. However, the Teamsters are much more than Hoffa or the mob; they are iconic in the history of labor reform in the United States in the 20th century, and can rightly claim a share of credit in bringing about collective bargaining rights for workers -- the cornerstone of unionization and worker's rights.
Membership: 1.4 million
Sticking it to the man: Teamsters in Minneapolis staged a massive strike in 1934 that exploded into violence, leading the state governor to declare martial law and call in the National Guard. But the Teamsters held out and won, and the strike would inspire passage of both the National Labor Relations Act and the Fair Labor Standards Act.