It's a rather vague question, but I'll give you some pointers / important information to consider.

Run EnergyPlus in Parallel:

EnergyPlus is a multi-thread application but is not optimized for parallel runs on multi-core machines. However, multiple parametric runs can be launched in parallel if these runs are independent

So if you want to run a fast single simulation, you're better off picking a CPU that has a high clock speed, regardless of the number of cores/threads (since it'll use only one anyways). If you are going to run a lot of parametric simulations, then you would be better off with a CPU that has a larger number of cores/thread even if the clock speed per core is lower. In this event, a larger amount of RAM would be preferable as well.

For I/O operations, running the simulation on a SSD drive is probably better though I haven't measured the impact it has (and would speculate it's minimal).

Also take in consideration what other tasks you're going to be doing. For example, how will you create the geometry? If you use a 3D modeling program (for eg Sketchup), then having a dedicated GPU would probably be a good idea to be able to manipulate larger building geometry without lagging.