Let's start with the 'Haiku Stairs' trail in Hawaii.  This illegal stairway to heaven on the island of Oahu is a narrow, rusty staircase that weaves up through the steep ridge-line in the Hawaiian mountains.  Perhaps the most dangerous part of this climb is the risk to your bank account.  Until recently, it was difficult to find and access these stairs (until I created a comprehensive post describing the method...).  Now the police seem to be cracking down on trespassers, threatening detainment and thousand-dollar fines.  

The Half Dome trail is hard enough by itself at 16 miles round trip (26 Km).  Once you run out of food, water and stamina, it's time to start the dangerous climb as you stand below the towering Half Dome cliff-face, alone as the sunlight fades into the darkness behind it.  This climb is nothing like it appears in pictures or gopro video.  It is much more intense, janky and dangerous than I had thought before I ever saw it first-hand.  If you slip your footing or lose grip on the thick steel cable and fall, you're dead as dead is dead.  The cliff you are climbing is much taller than the Burj Khalifa in Dubai which will give you plenty of time to watch your life flash before your eyes as you fall to the bottom.  This is not a hike for you mother, your father, your family members, anyone who has a slight fear of heights, or anyone who isn't completely confident in their climbing/hiking skills.  If you climb half dome and it starts to rain after you made it up, you probably won't make it down alive, and if you stay up there, you'll have to avoid being hit by lightning.  Bring gloves to get a better grip on the cable. 

There are several dangerous aspects to climbing Mount Rainier.  Crevasses, falling boulders and slippery ledges.  Mount Rainier is a training-ground for Everest expeditions because of the similarities the two mountains share.  

Crevasses:  There are crevasses you can see and climb over, but the real dangerous crevasses are the ones you can't see.  Sometimes these massive cracks in the glacier are blown over with snow.  At any moment you could be walking over one of these crevasses and fall through the snow to your death.  This is why the climbers on Mount Rainier are all on rope teams, connected to their buddies so they can be saved from a crevasse fall.

There is a rocky, cliff section of the Rainier climb.  On this part of the mountain, there are often rope teams above or below you.  The rocks are very lose and often accidentally or negligently slip out from under a person's feet and crash heavily hundreds of feet down near or onto the people below.  This is one reason to wear a climbing helmet during your climb.  One rock bigger than a bowling ball fell a couple hundred feet through the air and thudded 10 feet in front of us, directly on the trail.  (Helmet would be useless for that one!)

There's another part of the climb where you and you rope-crew are marching across a narrow path through the snow on a glacier.  The path is about a foot wide and flat, carved out with footsteps.  However, this narrow snow path is in the middle of a glacier that is about 45 degrees sideways.  If you trip over your rope, or stumble with your crampons and slip to your left a couple inches, the icy, slanted snow face would send you and your rope team sliding down to your left.  Now you and your team would have about 5 seconds of sliding time to stab your ice axes into the ice and stop yourselves from sliding off the edge of a massive cliff drop to your deaths.  

You should only climb mount Rainier if you are sure-footed and confident in what you're doing. 

Kalalau trail is especially hard if you are carrying a heavy pack.  I was carrying a bunch of water, tent equipment and heavy camera gear which made the trail a lot more difficult.  It was also super hot, humid and exposed... but beautiful.  This is probably the most other-worldly place I have ever been, I literally felt like I was in a movie.. a character on another planet with different rules and a different soul.  This hike is dangerous for a few reasons.  

1.  It is so long.  22 miles round trip.  That wouldn't be that hard on a paved road... But the Kalalau trail is anything but a paved road.  Over half of the trail is wet, slippery jagged rocks.  The other half is so hot and taxing that you end up drinking all your water and have to collect more from various waterfalls and streams along the way.  
2. You run out of food.  At least I did.  Sucks to suck. 
3. Cliffs.  This may seem redundant in all these posts, but there is certain section of the Kalalau trail that also cannot be described in picture or video.  "Crawler's Ledge."  I'm not being dramatic when I say this is dangerous.  Hundreds of feet above the crashing waves, this cliff-ledge-trail is 3 inches wide at some points and slanted downwards.  If it rains, this trail is literally un-passable and you will be stuck at the beach until the weather dries up.  You can try your best to pass in the rain but you will likely slip off to your death.  After the 94th person died on the Kalalau trail, they kinda stopped counting... 
4. Exposure.  The whole trail is up and down and up and down, thousands of feet in elevation change across the cliff and jungle edges of the "Garden Island" of Hawaii.  There is no escape to your left, just massive, sky-scraping (unclimbable) mountains, and nothing to your right but a thousand-foot drop to the deep blue ocean full of crashing waves and humpback whales.  Because of the mountains there is zero cell phone service and even if there were, once you get beyond a certain mile marker, it is illegal for helicopters to land.  The only thing getting you back to safety is your two feet and the trail.  So try not to break a leg... 

   Things you NEED on this trail:  Good Boots:  http://amzn.to/2h7wKYi      and Water / iodine tablets:  http://amzn.to/2heCqjQ

Everest Base Camp Trek is the hardest hike on the list.  

80 Miles through the Himalayas.  Alone.  No Guide.  No Porter.  Just you, your heavy pack, and your own determination.  On this trip I fought hunger, thirst, freezing temperatures, sharp knee pain, leg pain, altitude sickness, brain swelling, inner eye blind spots and for about 2 minutes I was in a coma.  (Read that story here...)  But if you are looking for a true challenge and true beauty that cannot be put into words, the best place to find it is deep within the Himalayas on the trail to Mount Everest.