Xenia. I don't have any in my tank, and having owned it in the past, I know the invasive pest it very easily becomes. BUT... if I could only have one coral in a tank, I think pulsing Xenia growing on everything would look pretty cool, especially with some clowns hosting in it.
I built the tank in June, 2010 and stocked it with a single 5 polyp zoa frag in December, 2010. This month's polyp count was 2502 and my plan is to cover all rock work so it will look like a single colony.
I have seen the gsp tank here and one other zoa tank that took a few years on youtube.