Always depended on the exact time period in my opinion. . Nick was early to mid 90's (Salute Your Shorts, Hey Dude, Clarrissa Explains it All, Real Monsters, Rugrats, Rocco's Modern Life, All That). Cartoon network when Dexter's, powerpuff, Johnny Bravo, Cowardly, Ed Edd and Eddie started mid to late 90's. Disney early 2000's (when everyone started getting it for the first time).
Can't choose one without the other. All of them. But I watched Cartoon Network and Disney mostly because it had all of my favorite cartoons, live action shows and anime on it. Nickelodeon is still my favorite for the classics. And yes, I still watch all these channels to this very day. :)
This is such a hard question to answer. I went through periods of liking and disliking one over the other, between Nickelodeon and Disney. Cartoon Network was bomb too though. Out of all of them, I guess I'd have to go with Nickelodeon having the best shows.