For f2p, power mining iron is the way to go. In the Dwarven Caverns (or whatever they're called), you can also mine iron outside of the resource dungeon and bank it inside for faster exp.
Preferably going for money. Im currently mining at Rimmington and banking all the ore at the dock using rsbuddy. Note sure where the dwarven cavern or resource dungeon is but ill look for it. Thanks.