Depends on what you mean by "adult fans". If you mean avid fans who go out of their way to watch the entire show, buy DVDs, and in general interact with other fans, then Avatar takes it. If you mean fans who just enjoy sitting down and watching the show whenever it's on, Spongebob takes it.
Avatar certainly does have appeal for adults. My dad, who couldn't be troubled to watch cartoons normally and said "You're seventeen, Jason" once when I was watching Naruto, became interested enough in Avatar to watch episodes I bought on DVD and then watch Sozin's Comet with my brothers and
me.
EDIT: Personally I totally disagree with the assertion that an episode of Avatar is worth only one viewing. While you can't get the same story anticipation twice, repeated viewings allow you to catch details and events you didn't really notice before (examples being Zuko's broadswords sitting on the wall of his cabin before the Blue Spirit, Azula being in the crowd at Zuko and Ozai's Agni Kai, etc.). Not to mention fight scenes are fun to rewatch, and Avatar has its fair share of humor that still gets a chuckle second time around.