Q: Which stats are good for combat?
A: At high gear levels, ArPen usually takes over; until you get there, it can be almost anything. I've seen Agi, AP, Hit, and Haste all take the top position depending on the exact details of your gear setup - check a spreadsheet to find the answer that's right for you. But by late ICC gear levels, the rule of thumb seems to be enough Exp and Hit to fix any crit cap issues you may have and then stack ArPen. In some gear setups you might find it advantageous to use a haste gem or two.