LapisScarab - You accept darkness, yet choose to live in the light. So why is it that you loathe us who teeter on the edge of nothing? We who were turned away by both light and dark - never given a choice? TALK - That may be... however, what other choice might we have had?
Any of Xemnas' monologues or soliloquies. They are simply impressive to listen to, partly because of his awesome voice.
A close second is Roxas' "Who am I going to have ice cream with?", because it was the single most (unintentionally) funny line in the series. I know that was supposed to be a really sad scene, and it is, but the way Roxas says that line was so serious that I could only think "Dude, priorities!" the first time I heard it.