what kind of lighting do you have? I can't really recomend a coral to you without knowing the lighting conditions.. I would say a hardy coral for just about any lighting condion would be some button polyps,some mushrooms,or maybe a pink toadstoll leather, these are all hardy and easy to keep. The most aggressive coral you will ever run in to is the galaxey, some of the other aggressive ones are hammer,torch,frogspawn,octo,bubble and a few others, basicly anything that has the ability to reach out will sting surrounding corals, you just have to be careful. BTW the aggressive corals I just listed are all HARD corals and require moderate to high lighting (VHO - Metal Halide)
Good Luck!