They are saying that of all the ORIGINAL FIRST RUN sitcoms on a BASIC cable network, "Tyler Perry" is the highest rated overall.
This means you discount all things like "Sex In The City," "Brothers," "It's Gary Shandling's Show," and all the other original comedies on HBO, Showtime or any other higher than basic tiered cable channel.
Also excluded are programs like dramas or sitcoms running in reruns like "Family Guy." This would also exclude things like "You Can't Do That On Television" which aired in Canada over the air (not a great example since it wasn't really a sitcom but you get the idea)
So that pretty much leaves what? The original TBS sitcoms like "Down To Earth" (which was pretty good). I imagine this would exclude shows like "Sponge Bob," which are not really sitcoms.
So what's left? I don't have cable so I don't know. Aren't there a couple of shows on the Disney Channel?