Since this is mainly a cycling forum, the results I see after I voted really don't shock me.
I didn't think much of triathalon's until I went and watched a few friends compete in one. Well actually it was a duathalon, but I can only imagine tri's being that much harder. So I went home and tried it. I went out and rode 20 miles on my bike at a 22mph pace, then I tried to run. What a joke. After about mile 2 my heart rate was through the roof, and I wasn't going much faster than 5 or 6 miles an hour. My legs ached bad, and I'm thinking, oh my gosh these guys run for 6 miles, bike for 25, then run for another 3 or so.
It's easier to get good at one thing and perfect your position, technique. Now triple the time it takes to get good at one, and add swimming and running.
I think the ironman is a harder event in terms of mental toughness and preparation to win than the tour is. 3 events compared to 1. But then one could argue that the tour is 3 events also, team effort, time trialing, and climbing. I voted for ironman.