Well, any list of PKers would have to include Foray as the obligatory first entry. He killed lots of folks and frequently - and over a very long span of time. I think that combination puts him in the lead.
As a player, I was partial to Munster. He had multiple-personalities and that always kept you off-balance. One day, he’d help equip you; then the very next, you’d find a Perle Svaerd in your head.
I worked in a guild with Carrot, which was fun. He was less interested in truly PKing people, and more interested in just hassling them. He was the kind of guy that would Crystal Ball you, see that you were fighting Chancel, and then wall you in with him!