The i7 3930k is by far the most useful cpu at 6c/12t but it's also a 130w lga2011 cpu so will require a very good cpu cooler (don't even think about budget cheapo coolers like hyper212) and an lga2011 motherboard, which aren't cheap and are getting harder to find.
For mainstream motherboards, the lga1155 i7-3770 would be better, even if it looses out a little to the i5-4670k in some games. It's getting to the point that 8 threads is more useful than 4 in many games, like WoW or gta:V or any other open world, high server drop games like fortnite.