I like Nihilus, to me he is the SW epitome of the ultimate power of the dark side and the what ultimately the dark side would take from you. That constant struggle between the dark side and humanity.
Clearly Malachor V hit him extremely hard, and twisted his connection to the force. Kreia seized upon this, and twisted it further in order to control him through his 'addiction', teaching him ways to feed his hunger. This leads into a downward spiral she couldn't even predict the depths of, culminating in his physical body becoming something almost ethereal, literally on a knife edge between physical being and an almost living force ghost.
Which I think ties in with his extreme powers. It's as if he can see beyond the 'mortal' realm and see into the infinity that is the force like a force ghost could, and thus he can tap into it and control it more than others, albeit at a terrible price to his mind and body. It's a weird power, and juxtaposes brilliantly to the Exiles powers.