How tough an Ironman race is subjective. A lot of factors come into play when making that call. Albeit all boast the same distance, 140.6 miles (2.4m swim, 112m bike, 26.2m run), conditions and courses vary, making each course unique in its challenges and degree of difficulty. Even within the same race, at different times, the course may seem more or less difficult than in years past.
So how do you rank the races by degree of difficulty?
Raymond Britt, a veteran Ironman triathlete and Boston Marathoner, has researched and ranked the top 25 toughest Ironman Races based on average "finishers time". His results, and detail explanations of the research, can be viewed in detail here.
In the mean time, here are a few snapshots of his findings.
This image shows the average finish time for the 112 mile bike course.
This image shows the average finish time for the 26.2 run course.
I have always contended that: If you start choosing a race because of degree of difficulty, swim course, bike course or run course, because of historical weather conditions or because of any other factor you may think proper and important, you're never going to find the "perfect" race.
Just pick the one you want to do, for whatever reasons you may have. Make sure these reasons are yours and yours alone, then train for the worst and hope for the best.
I have my reasons for choosing Ironman Louisville as my Ironman of choice. And to be brutally honest with you... I hate swimming in the ocean! I have heard everything from "you're nuts, crazy, out of your mind" to "I love that course" and everything in between. Again, everyone has their own opinions.
Choosing a "tough" race doesn't make it any harder. 140.6 miles are 140.6 miles in any language, any venue, any country...any way you look at it.