Violet. Not ultra violet though. Violet looks good, but ultraviolet is another level that none of them have.
The thing with ultra violet is that the human eye cannot see ultraviolet light well, if at all, so when the corals fluoresce under good ultra violet light it gives a great pop effect since there is much greater contrast with the surroundings.
For example, if you crank up the violet light the corals fluoresce but everything in the tank is glowing purple. With ultra violet light the tank would appear dark, but the corals would shine bright.
Ultraviolet is great for aesthetic purposes for this reason.