Pikes Peak Trail, Colorado - the unpredictable weather and lightning bolts
Devil's Path, Catskill mountains - 14000 ft and 25 mile
Muir Snowfield Hiking Trail, Washington - Snowstorms
Mt Mckinley, Alaska - Avalanches and landslides
Mist trail, Yosemite - Rain, mist, slippery rocks and a challenging last stretch!
Mount Washington, New Hampshire - strong winds, extreme cold