For me its the Heptathlon.
I know some will disagree and there are dozens of other extremely hard disciplines but I believe this is the hardest because it has 7 completely different disciplines the athletes must learn to a suitable level.
From 200m sprint to distance running to the high jump to the the shot put and javelin to the hurdles. This is a great challenge IMO.
Thats why Jessica Ennis for me is the best.