Taylor Swift is a brand. She’s popular, because she’s smart. She’s built an empire for herself.
Adele and Christina are genuine singers. Aguilera’s vocal range blows my mind.
IMHO, acting goes before singing; acting goes before dance. Meaning that presentation is more than half of the performance. The best singers or dancers in the world, lacking stage presence, are not as entertaining as a good actor who is also a good singer or dancer. As an extreme example of what I mean check out the video below. Puddles' presentation is striking to say the least: he is graced with an unusually strong operatic voice, but many people are. What you'll remember him for, OTOH, is his mobile face and the acting ability to carry off the performance playing an absurd role to perfection. Halley Rehinhardt is a very competent singer too, but it's her acting ability that makes her stage presence remarkable.
Christina’s issue, from this humble commentator, is that she just doesn’t seem to have found her own thing. She clearly has the best voice of the crowd she came up with.
Adele writes her own material, and I saw her sing those big songs with nothing but an acoustic guitar accompaniment. And they sounded great.