Any full-wave 3D EM solver should work. Different softwares have different advantages, and are often decided by personal preference. Personally, I use HFSS (also known as Ansys Electronics Desktop) for simulating metamaterial unit cells.
You may wish to elaborate more on your specific needs, in order that we may recommend a particular software to you.
for unitcell mtm or amc, prs, ebd... the hfss is better and has more accurate results about the phase. but when you want use these unit cell in the other structures it depends on your application. for example cst has good redults about the gain of antenna. but, i think you use both of them. and import export from one to another one.