Fun question to answer and this actual question came up during a recent seminar Tom Wishon did for the British PGA.
There is no question the most important performance feature on an ironhead for all levels of players has to be its MOI, the head's ability to deliver as much distance as possible when the golfer hits the ball off center. If you look at the green design on most courses, far more trouble lies in the front areas and front side areas of the greens than over the greens. To be able to hit the ball off center and still have the ball fly far enough to get over a hazard, even if the ball only drops on the very front fringe or front side fringe of the green is far better than being short and in such a trouble area.
However, what can be a problem in putting MOI first in iron head model selection is that a high MOI may bring with it a head size and shape that simply is not pleasing to the golfer. So this brings in the second most important iron head model fitting requirement which is SHAPE AND STYLE AND THE OVERALL LOOK OF THE HEAD.
There is no question any average to serious player is not going to be able to hit the ball as consistently well with an iron he does not like the look of compared to an iron model that he really likes to look at when he sets the club down behind the ball. In other words, confidence born from the shape and style of the head is very important.
At the same time, having a significantly higher MOI is more important - so this leads to a statement Wishon came up with during this recent seminar that goes like this:
Among iron head models that you like the look of enough to be able to have confidence in setting down behind the ball, always choose the head model among those that has the highest possible MOI. ANd yes, sometimes the clubmaker might have to push the golfer into giving up some shape and style preferences to be able to do this.
For example, let's say the golfer loves the look of a pure blade muscleback iron. IMHO for any golfer with a handicap above 0, a pure blade is not a great choice because it has the absolute worst off center hit forgiveness. So in this case, you have to counsel the golfer about the importance of MOI and then hunt around to see if you can find an iron model that has some of the shape/style features of the blade but still has a good bit higher MOI. Maybe when you boil it down about the blade, more than likely the golfer likes the smaller than average blade size, the thin topline, the non offset hosel design the most.
In that case, getting him into a cavity back like the 555C would still give him most all these shape/style features but bring a far better MOI along with it. Or maybe you could even get him to agree to like the slightly larger head size of the 560MC so that the MOI and off center forgiveness could be even better. Compromise in the name of much better game improvement for the golfer, in other words.
For the less skilled golfer who does not have quite the list of "must haves" for the shape and style, here the sky is the limit for iron head selection. In this case, if the golfer has been losing distance lately from age, choosing either a high COR face iron or choosing a stronger loft iron design along with putting more hybrids into his set makeup can be a very wise decision for the iron model.
I don't really put center of gravity very high on the list for iron model selection. OK, for the less skilled golfer, sure, it won't hurt to choose a lower CG design, but not at the expense of choosing the highest MOI first and a high COR face after that. There just is not that much difference between vertical CG location among all the irons out there with a higher MOI.
Sole design can be a big one, especially if the golfer has a little more steep, downward angle of attack into the ball. In this case, a more rounded sole from face to back with a more blunt leading edge can help turn the slightly fat shot into one that is solid enough to get to the front of the green.
Bendability and the capability to add head weight can also play a role when the golfer's lie, length and shaft weight needs are a little more out of the ordinary.