Why do I think Carrie is the best technical singer in AI history? Because she is. Seriously, as a musician myself I've heard hundreds if not thousands of professional singers, and IMO she rates among the top 2%. Pure notes, pure vowels, near-perfect pitch, outstanding vocal control. She's not perfect -- her breath control is her one obvious weak spot; she sometimes swallows her words at the end of a line. Easily corrected with vocal lessons. She also sometimes oversings, but that's a problem she shares with 47 previous AI finalists. The one past AI contestant who might be her equal is Jennifer Roberts (and speaking of oversingers....)
Look, this doesn't mean I want her to win AI4. I don't. I think it's more than just a pure singing competition -- if it were, they could just sing scales for an hour every Tuesday. I agree with Warhol that you have to judge the total package. For that reason I prefer Bo, much prefered Nadia, slightly preferred Constantine, etc. But honestly, if I said I thought AI has ever had a better note-for-note singer than Carrie Underwood, I'd be lying. Whoever beats her has to overcome her enormous technical advantage and beat her with stage presence and personality. Those are just the facts.