Definitely the i7 2600K. Pair it with an ASUS Maximus IV Extreme and some high end Corsair/GSkill Sniper RAM, preferably on 1.5V and enjoy
Oh, and an Noctua NH-D14 to keep it cool when you OC, if you OC.
It depends what you need it for. A lot of gamers are opting for the i5-2500K and achieving a better overclock and/or less heat than the 2600K (fewer threads). And I doubt at those speeds you'll be wanting for the 4 extra threads.