I've always really liked Slayer, but over the last 2 months I've really grown fond of Dungeoneering.
The problem with Dungeoneering is that it can be a very hard skill to actually start learning and understanding. The Dung culture (particularly at the higher-leveled end of the spectrum) can be quite nasty, with people so caught up in their floors times & their own knowledge of the skill that anybody who is not on their level will cause them to have a temper tantrum and rage quit, or yell at you etc. As such, as a newbie approaching the skill, it can be very hard to actually get a grasp of the very complex concepts inside the dungeon - especially things like learning to key, pathing & map prediction, learning how to communicate among a team and work together quickly & efficiently.
I've been learning a lot over the last few months Dunging with friends and other people, and I've really started to love & appreciate the complexity of the Dungeoneering mini-game. It is like a whole entire world of its own inside RuneScape