I like how you don't need to work hard and earn cash for items to use there. Everything is made inside, so its really good for me (I'm slightly lazy when it comes to money grinding-sorta thing )
My favorite is smithing. As long as you have a lot of gold you don't have to mine, so just buy the ores, make the bars and smith the stuff. You can make tons once you hit a high enough level to make addy gear. Once you can make rune you can make even more! As long as you can mine it quickly or you are able to buy it, you can make a lot of money. I haven't gotten a skill cape with this yet because it takes awhile but I'm pretty close.
I would definitely have to say slayer. While it is not fun to train, it is very exciting when you get to fight monsters that require a high slayer level and wait for one of those wealthy drops.