The best bet is probably Foster's, being the only show to come up at all for CN (and coming up twice) in teh 6-11 demographic for one particular week. In a sea of Nickelodeon shows such as Spongebob and FOP, that's pretty good for CNUS.
As for the other top-rated shows... we haven't the foggiest. Like Sketch said, CN doesn't release their ratings in comparison to other networks, so it's near impossible to tell if one is doing considerably better than the other.
Honestly, some of the shows that aired (or currently airing) on Cartoon Network don't even warrant the TV-PG rating, I'm talking shows like Teen Titans Go!, DC Super Hero Girls, Clarence, Uncle Grandpa for example. TV-Y7 at least.