Without a doubt, one that has Union Representation.
"AGMA represents the dancers of New York City Ballet, American Ballet Theatre, Alvin Ailey, Atlanta Ballet, Ballet Hispanico, BalletMet, Ballet San Jose, Ballet Tech, Ballet West, Boston Ballet, Cincinnati Ballet, Colorado Ballet, Dance Theater of Harlem, Houston Ballet, Joffrey Ballet, Kansas City Ballet, Martha Graham, Merce Cunningham, Milwaukee Ballet, Pacific Northwest Ballet, Pennsylvania Ballet, Pittsburgh Ballet, Tulsa Ballet, the Washington Ballet, and the Corps de Ballet at the Metropolitan Opera, New York City Opera, Lyric Opera of Chicago, Los Angeles Opera, San Francisco Opera, The Washington Opera, and the dancers of Movin' Out." Wikipedia.
Why? Because the other ones don't. The best you're going to get from the rest is, Workman's Compensation. If you want more insurance coverage that's up to you. But it will be from your own pocket.
Analysis of Ballet injuries and financial costs.
How much do Ballet dancers make?
The average salary of a Corps de Ballet.