CN Vs. Nick Vs. Disney, eh?
CN > Nick > Disney. This is assuming total lifetime, and not just a certain period.
CN Wins because of the greater variety, with an unfair advantage in Adult Swim. It's had a slew of great originals that still hold up incredibly well like Power Puff Girls, Dexter's Lab, Courage the Cowardly Dog and Ed, Edd& Eddy. Other era shows like Cow & Chicken, Johnny Bravo, and Billy & Mandy where great two, but a tier below those for me. Modern CN had a lot of good shows, but to me they started to wear out over time... Adventure Time's first couple seasons where great, but then it went too weird for my taste forsaking the simple comedy. Regular Show I also got tired of after awhile. There was that period of many years where they sucked a lot, with Flapjack being the only standout of the time. Adult Swim is really what pushes them over, not just with all the great programming from other networks, but originals like Aqua Teen Hunger Force, Robot Chicken, Venture Bros, Rocky & Morty.
Nick is second, but it has the most stand out show overall. Spongebob Squarepants, the first 3 seasons, are quite possibly the best seasons of any cartoon ever. It had the scientific perfect mix of humor, intelligence, sneaky adult references and heart. There is perhaps no cartoon I can go back to and still laugh at as much. The later seasons got a lot less clever and funny, but there are still some standout moments. Another show that was grade A for it's first few seasons was Fairly Odd Parents. It crashed hard with THREE Poochies in the later seasons, but those first few where pure gold. Other than that you had other classic Nicktoons like Rocko's Modern Life, Hey Arnold!, Rugrats, Invader Zim and Jimmy Neutron. I'll even admit to chuckling at Drake & Josh a few times despite finding most live action stuff (even the classics) not great. Overall, less variety in classics than CN, but close because of the Godlike Spongebob.
Disney is a solid last because of a giant slice of time where nothing interesting really happened. In the late 80's early 90's you had the Disney greats like Ducktales, Goof Troop, etc. but after this period you had more than a decade of largely crappy live action shows. It wasn't until much later you got Phineas & Ferb which I only consider so-so, but gave the network some relevance. It was really Gravity Falls that put them on the map for real, and it lasted a short time... by design in a show of true artistic integrity, but it was still sad to see it go. Other stuff like Wander Over Yander and Star Vs. the Force of Evil seem like they are above average, but never pulled me in enough to check them out extensively. Disney can't compete.
