Transformation spells do not work. Least physical ones. There is an response to this that goes : the amount of energy that would be needed to cause transformation magically if it were possible, would be beyond the control of humans. Even if we could, the voltage and wattage in theory would fry you alive. On a molecular level, the cells of the body possibly would burst, leaking cytoplasm all over the place. And, besides Magick is about the subtle changes in the world, not the major ones.
Spells aren't measured on how powerful they are, but ralther how successful they are, at least I do personally. In light of the stated behind, the most successful spell I've done is a healing spell.