Bart isn't that particularly interesting or unique as a character. Stewie is really over-the-top by what exactly he's written out to be, but it works for the most part. He also has the most witty lines.
Bart is an iconic character who--at his apex--was probably as popular as any television character ever. When the Simpsons were huge in the early '90s, it was Bart that you saw on t-shirts, magazines, coffee mugs, bumper stickers, records, video games, etc. And while the character is clearly based on previous characters, like Dennis the Menace, he's more of an homage than a blatant rip-off.