Hmmm, this is such a tough question. I've been debating how to answer it, since this thread began...
As I did with the favorite/least favorite fish thread, I'll limit this one to corals I actually have, leaving out corals I do not have yet.
Least favorite first, because this one's easy. I've got some mushrooms, in our 200 Deep Dimension, that I'd really love to get out. When I set up the 200, moving everything from our 75, I was very cautious to not transfer over any of the soft corals from the 75. Several months after setting up the 200, I noticed a couple mushrooms growing, on the largest rock in the tank, which is also under some other very large rocks. Unfortunately, these couple mushrooms did what they do best and SPREAD! Grrrr
Favorite....hmmmm.....
This one is really tough...
My favorite could be my Oregon Tort colony or my ORA Hawkins Echinata, or among a couple zoanthids or a couple Echinos that I have. That's as narrowed down as I can get right now...lol