The one you build yourself will be the best...
SSD drives seem to be really good for online gaming as well...
Basically, it all comes down to standards. My MacBook Pro can "play" games on it's integrated 8600M, but I wouldn't say it's great for playing anything over 720p on low-medium settings at a normally 20 or below framerate. Most "All in One" pcs have Core 2 Duos or i series dual cores that are clocked to run in a notebook, so they have slow clocks. Ram normally maxes out at 4GB, which isn't bad, but it is expensive laptop memory. So if you want high end gaming at 1080p, no. If you want low-med graphics at 720p, likely yes.
My 2009 iMac 21.5" plays everything I throw at it...well games I'm interested in.
Core2duo 3.06ghz, 6gb 1066mhz ram and a Radeon 4670.
Star craft 2 on max, counterstrike at max, portal, c&c3..erm....company of heroes not so good at medium.
So I dare say a brand new iMac with corei3 or i5 and a Radeon 5xxx and 1333mhz ram would be a lot better!
Sometimes some games can look just as good at high as they do at max. As long as its playable and doesn't look like ass then you're good to go 🙂