The answer is ballet, as it takes years of training and requires over 10 hours a week of studio practice from about the age of 9, up to 20+ hours at the advanced levels. To a well-trained ballet dancer, all the other dance styles mentioned by others are easy.
P.S. The fact that we've got an Engineer equating Dancing with "Life itself," shows why this category receives the most "abuse" in the form of miscategorized questions. This category is not a catchall for every question under the sun!
As a dancer for 15 years, I can say breakdancing and cheerdancing. Because it is combinations of different dances and gymnastics. The others are quite fine, you really have to practice like ballet and the others. Basically dancing is an art so it should be practice and your talents must be sharpen regularly.
Are we talking learning the steps or doing it very well? Most dances are basically easy to learn the basic steps to, but if I had to name one dance that is just evil, it would be hip-hop...all but impossible, the movements have to flow from somewhere I know nothing about.
If I had to name a genre of dance that is the hardest, it would be Ballroom...very difficult to do well, takes years of lessons and years of constant practice. And, there are so damned many dances inside the shell of Ballroom...very hard style to get done well.
The Choreography of two people who have just moved in together and trying to navigate in the kitchen....hence "The Kitchen Dance..." ~ Tho once mastered, it is a Tango unto itself.
*wink*