If you want to absolute 'best' (Which is subjective), then the 8-Core FX-8320 fits the bill.
However, the 6-Core FX-6300 would also serve you very well, and it would run quite a bit cooler, giving you more overclocking room (If you wish to pursue that route). It's also about $30 cheaper, and you're unlikely to notice the difference between it or the 8320 in-game.
With either, you'll want an aftermarket cooler, such as the Coolermaster Hyper 212+ or Evo. The stock coolers simply don't cut it.
The 8-Core may also get a bit more of an advantage with future games that take advantage of multi-core CPU's, but I still think the 6300 would be an excellent choice.