The Titleist AP1 and AP2 are both very solid irons. The biggest difference that you will find between them is the amount of offset, with the AP1 having a bit more. The top line of the AP2 is also a bit thinner. Without doing a fitting, I would usually start a higher handicap player with the AP1 as that is really the "player improvement" club in the Titleist line. If the player was a bit stronger and in the 10 - 18 handicap range I would look at the AP2.
I you are an more accomplished player, say 5 - 14 handicap and like the look of a more traditional forged club then the CB is where to look. For the very accomplished player, the MB forged blade may work.
There are also a number of ways to "split" your set to get the best of both worlds; playability and forgiveness. I have someone that plays a split set of 4,5, and 6 in the AP2 and 7,8,9, and W in the CB. Devind and I both play 4,5, and 6 in the CB and 7,8,9, and W in the MB.
Find a fitter, most a pretty reasonable especially if you order the clubs from them. You fitter, along with your PGA Professional, will become your best friends on your way to game improvement.