If we're allowed to include non-Korean forms (kata), I would say the Pangainoon/Uechi version of Sanchin kata. It is the first learned in the Pang/Uechi art, the one most often used in the physical portion of Dan testing (often with up to four people striking & kicking you while performing the kata) and is a solid measure of body conditioning, strength and endurance. I honestly could not tell you how many times I've performed this kata.
As a side note, I prefer the training methodology used in China during that era for myself and my students. For example, Uechi Kanbun Sensei trained for several months on just the opening movements of this kata and I believe it took him a full year to learn the kata in its entirety. It took nine more years to learn just two more kata. This proved to be an extremely powerful way to learn kata. Once during a town festival he performed Sanseiryu so powerfully that no other martial arts instructor would follow him. I think the same methodology could be applied to the kata/forms of any art in that it can be performed with such power, such intensity, such focus that no one would wish to follow you (speaking of a competition/demonstration venue). Imagine taking a full year to just concentrate on one form, for several hours each day. What level could you raise to in such an undertaking? I think it is a rewarding way to train personally. YMMV