I would says she probably is the best, definitely the most consistent.  
I absolutely love it when she sings the old standards.

The close second that comes to mind was Tamyra.....I forget her last 
name.  But I remember that she did a version of Bacharach's "This House 
Is Not A Home" that nearly had me in tears. 

Lastly, I just rented Dreamgirls....which I thought as a movie was 
really really a huge waste of time.  But there is no denying that 
Jennifer Hudson can sing her ass off.  But she seems to have her Aretha 
permanently set to 11 -- I'd almost prefer it if she'd dial it down a 
notch or two.

Marty