You want to know what makes money for Miners?

Fel Iron
Khorium
Cobalt

Sometimes you can count Mithril and Thorium in there, but with the glut of nodes added to Azeroth, most of the Vanilla ores aren't worth as much as they used to be (though you'll want to check your AH, of course).

Why, you ask? Due to the way Blizzard added expansion content, the later ores (Adamantium and Saronite) aren't as necessary, any longer. This is why the "beginner" ores for each expansion still bring in plenty of cash. I was selling Fel Iron for 95g a stack while leveling an alt recently, compared to the 20g I was lucky to get for Adamantium.

As always, check your AH at various times of day/night and during the week and on the weekend. You'll notice trends, and can sell at the right times to make the most gold.

Khorium? Khorium sells for astonishing amounts whenever it goes up because engineers want their mount. Since Blizzard hasn't added a transmute for this particular rare ore, and because so many people aren't mining in the Outlands anymore, it's even rarer, which pushes the price sky high. I normally sell each ore for anywhere from 50g to 200g - and they always sell within a day or two.