TF2, CS:S, and whatever else...all in Windows.
Even after being released on the Mac for several years, they still run like crap in OS X compared to Windows. In my experience on both a 2012 and 2011 iMac, in OS X both are almost unplayable at times, whereas in Windows frame rates are significantly higher and there aren't any huge lag spikes.
Other games I can't say I have any first hand experience with in OS X. However, I'd still run them in Windows.
Despite all this, the 2012 iMac is an extremely quiet machine when gaming. 2011 wasn't exactly loud, but the 2012 brings that to a whole new level (half of which is down to the use of 2.5" notebook drives rather than 3.5" desktop drives).