I don't play RS anymore. i quit a few years ago. But when I did play, my favorite was Herblore. yeah, it's expensive but i just think it is the funnest with the most badass cape.
Id say my faviroute is slayer, get to fight lots of different monsters and the idea of different tasks keeps you busy and you dont get bored as quick, and it becomes good money when you get to level 85 +.
Slayer!
It's the best skill in RS, because You always kill a lot of high level monster, and you cant be bored with this skill =)
And you get a lot of cash from it
99 Slayer ftw