Wow. I can't think of just one! A lot of the performances already listed on here (Teddy Bear/Memorial Address, I am..., End of the World CDL 2000-2001, Haru yo koi etc.) I love. Some of my other favorites are A Song for XX (A Ballads version) from the 2003 Music Fair (the same live as Haru yo koi) and Happy Ending and About You from AT05. Visually, I love monochrome and immature from her Concert Tour.
http://www.videoplayer.hu/videos/play/11939
This is a lovely version of Dearest that she performed at Christmas some years ago. It's different from any of the other orchestra versions of Dearest I've heard and is really quite touching.