People have mad some great suggestions here.
For one thing, green star polyps(GSP) are a very hardy coral and probably the most easiest to care for in my opinion. Another nice and easy coral to care for are Xenias. They grow like weeds and are beautiful, and they also do pretty well in "dirty" water.
One thing though, if you have the proper lighting, pretty much any coral that does not need target feedings of phyto or zooplankton is not that hard to take care of. I think the hardest parts of the coral game is actually placing them accordingly. Some coral like lower parts of the tank and some like it high up, closer to the light.
There's a couple more coral that are pretty easy to care for, and i'll list them when i remember their names.
Hope you find some you like!