My first choices would be Speed Racer and Star Blazers, but those were syndicated cartoons shown on weekdays after school. It is hard to remember any actual Saturday morning cartoon that I liked as much.

I do not count Looney Toons since those were originally shown in theaters. And I am not counting the live action series that also were shown on Saturday morning.

I guess Superfriends counts here since it was originally just on Saturday mornings for early seasons before going syndicated in later seasons.