What is the most physically-demanding martial art?

The physical demands of a martial art are defendant upon the practitioner, not the specific style. A karate kata can be done slowly and quietly, or it can be done at a more rapid pace with tension and power. The effort put into the practice of any martial art will be well worth it.

Harder arts that are more suited for the young and strong would include Krav Magna, Mauy Thai, and Brazilian JuJitsu. But anyone with the motivation and desire can become quite proficient at them.

I am training muay thai and it is extremely hard work.... but as mentioned it depends on the trainer and also on you how much enthusiasm you put into the tranining.

Southern Praying Mantis style Kung Fu. You don't know the meaning of hard until you try this.

There are martial arts which demand nearly acrobatic skills, this is the case of Brazilian Capoeira. (Like Christie and Eddy from tekken!)

Tae Kwon Do is heavily dependent on kicking and leg work. As the legs use the largest muscle groups a Tae Kwon Do workout will always leave you sweating.

Sip Pal Gi, Korean Kung Fu, is a very physically challenging martial arts. Very low positions and 180 degree rising spins are part of the style.