I picked Climbing Mount Everest because of the fact that they mark the easiest route upwards using the frozen dead bodies of the failed mountain climbers scattered on Mount Everest, and also the fact that freezing to death is one of the most painful ways to die.
This is hard to answer because I have loved doing or am fascinated by every one of them. If I had to pick, skydiving for least scary. And rafting as most challenging since it's the only one in that list that's the most fast-paced and may require quick thinking