Really good thread.....
I've had to debate it and initially chose Daydream simply because the global album and single sales were all huge. But there is more to someone's star power than just that. Look at Gaga - he was having good success with stuff like Poker Face but peaked in sales and media presense with Bad Romance. THEN, she was a megastar. To be at your apex you need to be basically be on top of EVERYTHING as a media personality, something she had in far greater terms by 2005 with TEOM.
With that, she had album and singles success, the mega comeback, the endorsement deals, a pretty positive media following and (finally) the acceptance of her as not only a talented artist but someone who was regarded as legend. I do think TEOM was the first time the media actually bestowed that kind of praise on her.
It was about the only era where she loved and in vogue by everyone.
And she was literally everyWHERE too.