It's not necessarily the best, just the one I thought, as a woman of science, you would appreciate. The cost of using it is high, but by using the brains of people in good condition--mentally and physically--you can supposedly cure any ailment. When I ran into it, the Koga were going to sacrifice ninja teenagers and young adults from their village as the ingredients.

[The "peak" of the human condition is the young, afterall.]

The best... Objectively, the "best" of the kinjutsu isn't really one. It's the Shinrabansho, which is above and beyond the five kinjutsu. It has the power to do anything as far as anyone knows. It's the "knowledge of everything in nature". Life, death, it knows it all.