Sicko (2006)

Michael Moore interviews medics and investigators from private health insurance companies who admit denying legitimate claims for the most spurious technical reasons. According to Moore, it’s a scandal that can be traced back to Richard Nixon.
Health care isn't healthcare; bureaucracy, the labyrinth of paperwork and all legal language about pre-existing conditions and denial of service make having coverage as much of a challenge as lacking it.

To prepare for the film, Moore used the Internet to solicit health-care horror stories, not just from the 47 million Americans who don’t have insurance but from those who do. So he travels around to a bunch of countries that already have socialized medicine to see how they work and shows us how France, England, Canada and Cuba actually help sick people instead of letting them wither and die for lack of health insurance.

I don't know if we can accept Michael Moore's selected anecdotes as "proof", but the movie is very funny: only Moore can talk about political issues and make you laugh until you cry !
You can buy Sicko.