I think like a lot of people of my generation, we started with what was being shown on TV which would have been things like Pokemon, Yu-Gi-Oh, Digimon, DBZ, Shin-Chan, Shaman King, Rave Master and the various Transformers anime they had at the time. As a kid, some of my favorite movies were Ghibli movies and the 2001 adaptation of Osamu Tezuka's Metropolis. I always knew they were Japanese, they were just so different but so captivating at the same time. For some reason though, maybe because I was so young at the time, I never had the urge to go deeper and find more.
It wasn't until around 2007 I started to branch out and go a bit deeper. I was a huge Naruto fan at the time and had been collecting the manga since the year prior (The anime still hadn't aired on TV where I lived at this point). Eventually it did start airing, but because I was so ahead with the story since I'd been reading the manga, I didn't bother watching it but I ran into another roadblock; No book shop, comic shop or anywhere that might stock manga had the next volume of Naruto I needed. That's when I decided to just go for it and started watching the anime online.
Coincidentally around this time as well, a TV channel called "AnimeCentral" popped up on my radar after hearing about it from some friends at school. I guess because of the type of series they showed they had to follow the TV guidelines in my country because the channel would only air from 9PM to 6AM and then it was just dead-air for everything in between those times. That channel was what really opened the gates for me though, that's where I saw stuff like Cowboy Bebop, Ghost in the Shell SAC, Planetes, Wolf's Rain, Escaflowne, Bleach, Fullmetal Alchemist (2003) as well as the Takara Transformers animes from the 80's. Unfortunately, that channel only lasted a year before closing, but because I already had knowledge of where to find anime online, it wasn't that big of a deal to me.
So I guess to actually answer the question, it would be Naruto and everything that aired on Anime Central.