Well, going off the dates, it looks like I would've started watching toward the end of the Powerhouse era and continued through City into the first couple years of Nood, after which we moved and didn't get the channel at all anymore. So it's probably a pretty firm tie between Powerhouse and City for me. I think I'll give City the vote, though, since it doesn't seem to be getting many.
Powerhouse because that is when we had several classic original cartoons such as Dexter’s Laboratory, Powerpuff Girls, And Courage the Cowardly Dog. They also aired animated movies and the original Toonami was huge for the network.
I know I was a huge fan of "Scooby-Doo Where Are You?" When I was a little kid. I would always ask my mom if I can stay up until 11pm because they would air 2 episodes back-to-back at 10pm. This was back in 1997. From there, originals such ask "Dexter's Laboratory" and "Powerpuff Girls" peaked my interest. And even watching them now as I am older, the double-meaning and hidden messages behind it crack me up but yet are not as obvious as "Adventure Time" makes it to be.