Ningen tte Sonna Mono Ne from her album "trip trip," I really like the piano rearrangement on this version. It's also interesting to note how her voice has changed throughout the years. Regardless, she sounds amazing.
My other favorite KOKIA Lives here:
http://www.youtube.com/playlist?list=PLukIoKUw4ng8cVKeEj-KqSRIlGl3_PiE0