I think you should consider color more than coral.
First off, I congratulate you, and not said sarcastically for posting a photo where the tank isn't awash in Windex blue.
Since you posted a more white enlightened tank, the range of colors in your tank is not strikingly dramatic. I would consider a coral (of any type) that is bright red or orange.
Not only will red or orange stand out against the black background and the rock work, but the such a dramatically different color than that of your existling livestock will make the greens and blues that you have now stand out even more.
In addition to the 'Big' addition you are looking for in the middle of the tank, be on the look out for some smaller bits of color to break up having too much of the greens and blues next to each other. Breaking up the blue/greens with little bits or orange, yellow, red will help make each coral pop a little more.
Lepastreas, Favias, Some of the radically colored Gonioporas all could really help boost the bounty of colors in your tank.
Dave B