From what I've heard, RAAM is one of the most exhausting race. It's a 5,000 kilometers long biking race across America. They start...and then who comes first. They sleep 20 minutes/night...almost every year someone dies...
I would say Formula 1. A F1 driver loses about 4kgs of his body weight in just 2 hours. And the mental strain and the need for quick reflexes makes it even more exhausting.
Shortly after moving to California I finished a marathon in under 3 hours and still didn't consider it as draining as high school wrestling. To be competitive in wrestling I needed to run 30 miles per week, bench press almost twice my weight and be able to do 30 chin ups. For the marathon it was 50 plus miles per week and some cross training. I almost never laugh out loud but I busted a gut laughing when some guy in the office said soccer was the most exhausting sport. He thought they ran at marathon pace for 90 minutes. I enjoyed informing him that only the top leagues cover more than 6 miles and so he was running less than a quarter of a marathon at a pace that would have him running a seven hour marathon. So my vote goes for the marathon, wrestling, and cycling events like Tour de whatever.