Hands down, Saint Seiya. Used to watch it all the time when I was little because my brothers are much older than me and loved the show. I watched little kids shows too, but Saint Seiya was special because it told such a good story that was bigger than what other shows for kids my age were doing. The show had everything, epic battles, friendship, sacrifice, drama. I grew up wanting to tell stories that hit those notes, the kind of stories that makes you feel and root for the characters. Saint Seiya had some pretty tragic backstories and I think that also made me enjoy writing characters that are damaged in some way. I still love the show, and I still love the story it told even if it wasn't perfect. It's also interesting to hear it inspired some other more famous shounen titles.
There were a bunch of other shows that influenced me as a kid, like Cowboy Bebop, Trigun, Inuyasha, Digimon, Sailor Moon, but Saint Seiya really started my love of storytelling.