Such a sweet question! Thanks for asking. For me, Britney's electricity comes from her creative will and artistic straightforwardness. You can say what you want about her, but the Britney I love is always really concerned about her voice, music, videos and performances. In other words, she's dedicated giving one hell of a show. The rest, her life and all, is peripheral. There's something about her undersized ego, you guys say it very well, that goes perfectly with that wildness I love. It's a delightful contrast. It still shows in her recent instagram posts: you can see she's simple, she's never pretentious, but she's bold.
She's intelligent enough to be self-conscious but it never shows in her behavior and art. Other divas (Gaga, Madonna, Rihanna) tend to externalise their self-consciousness; they brag and get unfriendly or remote. The main other star that makes me think of Britney's lightness of being is Céline Dion, but Céline is more about technical power and less about entertainment. Britney understands so well what we want. What a charm of a human being!