I don't know why, but I've always enjoyed seeing how a mangaka creates his villians. Togashi-san does very well in the villian department. Having the crazed psyco (Hisoka), the scary brother (Illumi) and a whole group of villians, (Spiders) all in one manga!
(There are probably more villians, I just haven't met them yet. I'm only up to manga number 12.)
Of course, main characters are vitally important as well.
Gon seems to catch my interest the most. I used to be an avid Kurapika-san fan, but now grow more and more fond of Gon.