First on television, that's all.
And Kimba was the first color anime but that doesn't make it great.
Astro Boy was pretty damn good for any anime, but Tezuka was better than that. Damn, did Adult Swim stop showing Astro Boy? I might consider watching it again. The original of course.
Oh, and did you know there's gonna be a CGI Astro Boy movie? Gah.