Anything by Urasawa, really. It was weaker in Billy Bat though.
His black clothes create an intense dichotomy between him and the deep white, yet gentle snow around him.
The metal on his arm and chest represents a steel will - the will to fight and survive.
His sombre black eyes show determination to fight for his loved ones.
The ice in the background convey his cold, yet gentle heart.
Lastly his black sword is the ultimate tool. Black to some may represent evil or dark, but to Kirito, the Black Swordsman, the black sword represents him repelling the evil that attacks him.
Truly a deep and misunderstood character.
>but to Kirito, the Black Swordsman
Haruhi's ribbon gives her a really nice touch.
In all seriousness, one of the few things SAO gets right is character design. Aside from the stupidity that comes from having an entirely black outfit, Kirito looks good.
YuYuYu's design have better color schemes and really showcase how well the abec/BUNBUN style can work.