That entirely depends on the money you are willing to spend. Within given priceranges, which brand has the better processor for gaming varies. If you have no budget, the i7 2600k by intel is the current most powerful CPU for gaming. I highly recommend you forget about brand and focus on "Which CPU is the best performing gaming CPU for my pricerange?" Tom's does and article on "Best gaming CPU for the $" each month and I, for the most part, agree with their assessment.
That said, in my opinion, the following are the best gaming CPUs within their given priceranges. Note that I didn't research much into current prices but generally the prices for each of these CPUs you find are worth it. I may also have left a few worthwhile CPUs out:
Athlon IIx2 3.0-3.2GHz
Athlon IIx3 2.9-3.1GHz
Athlon IIx4 2.9-3.0GHz
i3 2100
Phenom IIx4 955
i5 2300
i5 2500k
So as I said before, your budget is one of the most important variables ($budget) in determining your CPU. The other big variable is the video card you are using or intend to use ($VGA) and other, more minor variables are the screen resolution, games that will be played, if you plan to keep this CPU through a second video card upgrade and if there is a specific place of purchase ($rez, $games, $VGAup, $plocation).
So if you would like more specific help from us, please reassign the following variables:
budget = "$200"
VGA = "Radeon 6950 1GB"
rez = "1920x1080"
games = "modern and upcoming games"
VGAup = "yes"
plocation = "newegg.com"
So for example, in the above case, with prices from newegg.com, I would recommend an i5 2500k if you can find it on sale or are willing to pay the extra $20 over budget, the i5 2400 if you never intend to overclock, and the phenom IIx4 965 if you want to leave some room for taxes and shipping.
Personally, I overclock. If I were building a gaming system right now for myself, I would choose either an Athlon IIx3, Phenom II x4 955 or a core i5 2500k depending on the money I had to spend and the video card.