After receiving some feedback, here's what I've come up with as the hardest recreational endurance events in the world. My definition of recreational is that anyone may enter. You may have to qualify but that standard isn't based on natural ability. This eliminates obvious stuff, like riding the Tour de France but also things like age group world championships and races like Hawaii Ironman. I wanted to make a list that anyone reading could think "I want to try that" and then, with enough motivation, have a chance to do it.
This list is my first attempt and in no particular order as to difficulty. I've tried to encompass as many different diciplines as possible. All feedback is welcome.
Badwater 135 (or 146 since you SHOULD continue to the summit once you've come all this way) - From what I can tell, this is the world's must rugged foot race, mainly because it's held on what's supposed to be the hottest weekend of any particular year. In temps that nearly always exceed 120 degrees F, you run through the desert from the lowest point in the United States to the highest (in the lower 48). The entire race is on pavement, adding to its nastiness, since runners must stay on the white lines of paint to keep their shoes from melting. Oddly enough, this race is popular enough that it's hard to get in, so get creative with your resume.
Race Across America - This could also count as the most boring but I don't think anyone could argue its gruel factor. Just sitting on your bike for this long is daunting enough. But you've also got to pedal, and at a decent pace to stay in the race. Add lack of sleep in extreme heat, cold, snow, rain, wind and then throw in the occasional tornado and you've got the perfect recipe for sheer misery. Anyone can enter but you've got to complete a qualifier of, at least, 400 miles within a respectable time of the winner.
The Norseman - Billing itself as the world's toughest triathlon, it's certainly hard to argue. Only slightly longer than a traditional ironman, the course profile alone looks ridiculous. This is before being told that the swim is in 60 degree water and the run is a rocky ascent (no trail from what I can tell) to the summit of a mountain.
Iditarod Trail Invitational - Dogs? We don't need no stinking dogs! This human powered version of the famously miserable Iditarod really needs no description. It's Febuary in northern Alaska. It's dark. It's really really cold. And you get to ride your bike 1100 miles accross the ice. Good times.
The Hardrock 100 - The trail runners holy grail, this 100 mile jaunt across the top of the Rockies is a shoo-in to make the list. Many life long ulra runners never finish this extreme test of will that features nearly 70,000' of elevation change over rock and snow at an average of 11,000'. Record holder Karl Melzer calls it "the hardest ultra on the planet."
Climbing Mount Everest - Sure sure, K2 is heaps harder, as are many peaks, but none of those are available to the recreational athlete. But Everest, most famously, now is. If you're rich or good at schmoozing pretty much anyone can get themselves a shot at standing on the world's rooftop. But cash and cocktail partying aside, you've still got to get yourself up the sucker and no guide, fixed rope, oxygen tanks or Sherpa can do it for you. Most that try don't make it. Those that do come down changed. And some don't come down at all. Commercial as it gets, high altitude roulette is still a dangerous game.
Primal Quest - According to Rebecca Rusch, one of the sports elders, adventure racing ain't what it used to be. "I'd say, for sure, the old style Raid Galuouse would be on this list. In those days even the organizers didn't know what to expect. It was truly adventurous." But commercialism and a "safety first" attitude aside, AR is still a tough test of the limits we can endure. And none, currently, is tougher than the Primal Quest.
Crocodile Trophy - This 10 stage, 1400 kilometer stage race through the Austrailian Outback is the antithesis to the Iditarod race. The race crosses Oz through what is famously some of the most inhospitible terrain in the world. From rain drenching rain forest to the bleakest desert, the course profile doesn't begin to do this race justice. Most of the competitors are elite, if not pro, yet the most common quote heard throughout the race is "I expected it to be hard but completely underestimated it." Probably why the organizers have dubbed it "the toughest test on two wheels."