This Iron Ore farming guide will list the places where you can get the most Iron. It's recommended to have Mining skill 100 before you start farming Iron Ore, because you will get more ores.
Iron Ore
Playing Classic Era?
This guide is for Retail WoW and Cataclysm Classic. I have a separate guide for Classic Era:
Iron Ore farming in ClassicFeralas
You will find Iron anywhere you go in Feralas, it's the best place to mine Iron. You could actually just do laps at the northern area without going to the southern area.
Western Plaguelands
Western Plaguelands is also an excellent place to farm Iron, just don't go to Hearthglen because there is no ore there. Stick to route I recommend.
These are the best zones if you want to farm Iron Ore. Once you are done mining Iron, go to my Mithril Ore Farming page!