I, admittedly, don't watch a lot of these, so it would figure my pick would be this, no?
Timothy Treadwell may have been a littlle off his rocker, but his is a truly interesting story. His love for the bears (and foxes) is unmistakeable, but there's a weird duality in that his protecting the bears may have been doing more harm than good. But it just adds to the depth of the film, which is full of beautiful footage of the majestic grizzlies, and sometimes painful to watch monologues from Treadwell.
Murderball is a good runner up if bears aren't your thing.