It generally depends on what is the purpose of the PC. The answer is usually the:
i5-3570k for playing games.
i7-3770k for a mix of gaming, 3D modelling, video encoding/editing and heavy Photoshop usage.
i7 3970x is best for extremely CPU intensive tasks that can take advantage of 6 cores like 3D modelling and if money is generally no object. Sure you can still play games, if games is the only purpose then you are better off just buying an i5-3570k and using the rest of the money on a high end graphics card.