The pking is probably my favorite, although as someone already said, it has to be a well-coded server otherwise it can be pretty over-powered and lame.
I love the communities on most private servers too, you tend to get a lot of the decent guys that've just tired of RuneScape and have moved onto Private Servers.
I don't like ::master or :

ure servers that much though tbh, I prefer to work for stats a little, but not so much that it's as hard as RS, that kind of defies the point of a Private Server.