Slayer. All you do is attack stuff and you get exp. But it does take a long time to get, it is rather easy. Lots of skills are easy, such as fishing, woodcutting, cooking (if using a stove), fletching (making bows u), herblore, prayer, farming. To make a long list short, most of the skills in RS are easy. Don't let something thats easy confuse it with something thats quick/cheaper to do.