Agreed. Unless you're ridiculously lucky with slayer drops. An argument could be made with things like chins, mage logs and rune ore/bars - but I think you'll get the best profit per hour with RC'ing.
Ye mining is a pretty tough and long skill but in the end it will pay off and its a skill that probably will never get old and does not take any gold what so ever to skill in.
Runecrafting is the best if you want to talk about consistancy, especially once you get high level. However, slayer could easily overrun it but it's 110%+ based on luck...
runecrafting - most profitable skill
slayer - u can get good loots like whips and stuff
mining - mine ores , sell em or smelt em more profit
fishing - fish and cook/sell for good money
hunter - hunt red chins veryy good money
wc - yews/maples/mage logs
thieving - pyramid plunder may get u the staff
smithing - make cannonballs or something profitable
fletching - make longbow (u)s and string em sell for good money
RuneCrafting by far, you will get a huge amount of profit. However the hours spent in game with any money making method isn't worth it, the only good money making methods are staking and merchanting on a large scale.
As stated by many, runecrafting is by far your best option. 100% consistency with regards to income; double nats at 91 (I believe). But be prepared for one hell of a boring time..
Depends on your method. I'm working towards 99 currently. Only 67 at the moment, but from 38-67 I've easily made profit. Just takes a little patience to buy the ingredients for the right price, and sell on for the right price. Also takes a fair bit of cash to be able to buy in bulk to begin with. So unless you're willing to collect your own herbs, it's one of those "need money to make money" dealios.