Spongebob. For many reasons. It came out without anything comparable to it really. Actually when the first "mysterious" commercials came out for SB, it had me... questioning.
At first I said, uh-oh, this doesn't sound good. It probably won't last for more than 6 months. I mean, who would be stupid enough to make a show called "SPONGE-BOB"? How fun could a show about a dish sponge named "Bob" be?
My first vision of the show was some sort of a rehash of Stick-Stickly. (Stickly wasn't "horrible", but who'd want to stick around to see a rehash of 'em? Note: Pun not intended.)
But then I was impressed that SB actually pulled it all off, against all odds. And not only that, but continued to become a worldwide phenomena. If I draw Spongebob on a piece of paper anywhere in the world, I'll probably find a good handful of kids in the crowd that knows who he is.
Whereas Flap Jack, came in years later as a Cartoon Network knock off in their desperation to match SB's fame.
You take the character of "Spongebob", animate his show with the same studio that did "Chowder", invite the eerie minds of "Billy & Mandy" and "Courage the Cowardly Dog" to help out with certain details, and pepper in some the utterly stupid insanity from "Ren & Stimpy" and out comes your brand new baby "Flapjack".