I interpreted the ultimate coordinator concept in a completely different manner. They explained that normal coordinators aren't really "complete", so to speak. Due to the unpredictability of the mother's womb, not all of their genes can be successfully altered. So even among coordinators, some may have better resistance to certian diseases than others, or some may have a higher tolerance to various tempatures than others. This was shown when they tried to give coordinators specific physical traits at birth, such as blond hair or green eyes. They were still coordinators but with a few small differences in the 'coordinator' genetics. Kira on the other hand was born from an artificial womb that was designed to create a 'perfect' coordinator who didn't have any such flaws that the mother's womb could create. It doesn't necessarily mean Kira is better in every way to any other coordinator, it just gives him small but notable advantages over other coordinators. Most of Kira's skill is something that he acquired on his own, and has little to do with being the "ultimate" coordinator. Thats how I like to think of it anyway.