This is mostly going to be personal preference, so people will likely suggest the pieces they like best.
I will say that the 2 peices in my tank that get the most oohs and aahs are my red goni and my pearl bubble (especially when its feeders are out).
I think a big favia can be really spectacular though. There are some really nice Lepto colorations. Same with acans and lobos, though I've not had much luck with them.