My vote for single easiest coral to keep alive is cabbage coral. I had a piece dissapear once, i found it over a month later, under a rock where it got zero light. It was still happy as can be, I re-attached it and all is still well (except my urchin decided to give it a taste test a few weeks ago).
Another ridiculously easy coral is capnella. It is similar in appearance to colt coral (may even be a type, i dunno). This stuff grows like a weed, and it doesn't have to be fragged, cause it frags itself when it gets big enough. I have it sprouting up all over my tank, and it's only been in there a few months.
My vote for easy and fairly attractive (I like the previous two mentioned but they aren't exactly show stoppers) is encrusting star polyps. They have several shades, I have some that is pink, some that is green and some that is blue and red. These aren't going to be centerpieces either, but I think they make great filler corals. They grow extremely quickly as well.
Pulsing xenia is a great choice, and is very easy once it is acclimated, but for the first month or so you have it, it's survival is hit and miss. Also, IME, they are among the most likely of all the corals to be munched on by other inhabitants.