Maybe this one?
http://www.youtube.com/watch?v=x0W4R_jJtV0
Everyone knows and loves "Dearest", and this is one of the most beautiful performances of this song that I can think of.
If not this, then "A Song for XX" at a-nation 2008 is probably one of her most iconic performances. Also "M" from A museum.
Of course, I also think that either of the "teddy bear" performances count.
Maybe?
