I have watched lots of documentaries since the 90s, and while I can't think off the top of my head which was the most interesting a few came to mind:
Tulsa: The Fire and the Forgotten (PBS) - This is something I had never heard about before watching it and I was shocked to see what took place took place in the Greenwood District of Tulsa, Oklahoma, USA from May 31-June 1, 1921. The Civil War, The West and The American Buffalo by Ken Burns
The Born Free Legacy (BBC 2010, not the 2011 Elsa's Legacy: The Born Free Story). This is the true story of George and Joy Adamson and their adopted lioness cub, Elsa, they looked after and returned to the wild in Africa and followed their lives and conservation efforts. The 1966 film, Born Free, was based on this true story.
I forgot to add these two to my original post, Ken Burns "Jazz" and "Country Music" I like both genres of music so I really liked both series, especially loved the Jazz series as I like listening to Jazz music to and playing Jazz music on my keyboard and Saxophones.
