For me its Slayer. Its fun on its own and around it I train other skills. Sometimes I range a task so ill be needing arrows, to make those I need mith, wood and feather... So while doing a task I train mining, smithing, woodcutting, fletching, range , prayer (from the bones),HP and slayer. And a bit of mage when theres drops to high alch. This works for me because I dont like doing the very same thing all day long. Almost forgot herblore! Collecting the herbs during the task... SLAYER!