Hi,
For reasonably low frequency (let say roughly 10GHz) any 2.5D or 3D planar simulator will be accurate enough. I did some simulations in the past and they were all giving more-less the same results. The simulators I tested were Sonnet, Momentum, IE3D, Esemble. The major difference when using them was the simulation time and ease of drawing the geometries in the drawing editor. Having everything in mind, I will recoment Sonnet as the best solution. The conclusion on this topic, however, depends a lot on a particular structure you are analizing.
For high fequency, I will recommand some full 3D solver like HFSS or CST. Again, the trade offs are depending on a your problem.
flyhigh