Much as the other networks have fallen off, this one for me was an easy choice: Cartoon Network by a country mile, with the various US broadcast TV networks tied for second and syndicated children's TV (yeah I know, "what?") as third...
But to throw out my thoughts on each one:
Disney Channel: Think it slid downhill once it got rid of the classic programming (which judging from Toon Disney is now unavailable *anywhere* on US TV...) to focus only on new/original programming. Still, some of the new stuff is OK, but do think they should rerun older material *somewhere* within Disney's massive media empire outlets (not just dumped on DVD with little promotion or fanfare, or only used as remakes a la "The Parent Trap").
Toon Disney: Only get to see this when visiting relatives out of town (I dont' have digital cable), but was pleased to see them start airing the Silver Age WB shows Time-Warner's apparently left for dead (Pinky and the Brain, Batman:TAS, Superman:TAS), even if that's off-topic (not Disney owned). Unfortunately, they also air junk like "Power Rangers" to death. At least some older Disney Afternoon stuff is (or was last I saw it) available (though oddly no DuckTales).
Cartoon Network: as everyone else has noted, this channel's mix of old and new entertainment (Looney Tunes and Hanna-Barbera stuff combined with new productions like PowerPuff Girls and Adult Swim) were its high point; unfortunately, things have slid to the current point of only airing the same several mediocre new cartoons over and over, along with a bunch of live-action junk that could easily air on one of the zillion other Time-Warner media outlets. Meanwhile, no Looney Tunes/theatrical shorts or older TV material (aside from Scooby Doo and the occasional Cartoon Cartoon compilation TV show). Adult Swim's also seemingly (to me) slid off quality-wise, though I only liked a few of the shows made for it, anyway. Easily makes my choice for this poll.
Boomerang: While I don't get this channel (no digital cable, though it's carried on my company's lineup), and I rarely get to see it (my relatives' cable company only carries the On Demand version), it's still miles above Cartoon Network quality-wise from what I can see---even despite sliding off lately quality-wise with Time-Warner using it as its dumping ground for axed CN series (JLU, Duck Dodgers, etc.) and trying to air a bit of live-action junk on *it* as well.
Nickelodeon: Most of the cartoons seem to have taken a powder lately in favor of a bunch of mediocre live-action shows about teenage musicians/kids-going-to-school being rerun to *DEATH*. Still, it's keeping with Nick's tradition of running originally-produced, live-action programming. Do wish that they hadn't given "6Teen" the bum's rush (a show that's now unavailable anywhere in the US)...
Nicktoons: Only see this one when visiting relatives, but like that it runs some older Nicktoons... unfortunately, it doesn't seem to run too many of them, apparently in favor of original productions and the same stuff (or what's left of it) still running on Nickeldoeon-proper. Wouldn't kill them to air more "Angry Beavers" ... ;-)
Other:
- I'll let our Canadian and UK (and elsewhere) friends fill us in on whether or not Teletoon/foreign versions of CN and Boomerang/etc. have gone downhill as well (to avoid this just being US-only)...
- Broadcast TV's slid off *big* time, to where it's pretty much all but dead (with 4Kids taking over Kids WB next fall probably the last nail in the coffin). Yes, they have E/I requirements, but the fact syndicated TV stations would rather air infomercials on weekday mornings vs. cartoons (or the ones available for syndication still) seems quite sad...
-B.