Theese are the reasons i think Spike is best for Buffy, please note theese are not in chronoligical order.

1)Spike was able to Cinta Buffy without having his soul.
2)He refused to tell Glory who the key was even after being tortured.
3)She treated him like a man even though he knew he didn't deserve it. "I know i'm a monster, but anda treat me like a man"
4)When Buffy's Friends where all in hospital and Kendra was dead, Spike was the only one who helped her.
5) He was the first one to know Buffy had been in heaven not hell.
6) Even when he would have been easy to kill with the chip, Buffy didn't kill...