My choice might seem a bit odd, but hear me out...
I know that seems like an odd choice consider how many albums she's sold. But, I feel she rarely gets the credit she deserves for being an ACTUAL TALENTED SINGER. When she first hit the scene in 1998, people kind of discredited her as a "Britney Spears Wanna" and was kind of always more or less in Spears' shadow, most likely because Spears hit it famous before Aguilera and maybe in some people's mind, is more "marketable." But no doubt about it, when it comes to actual singing, Aguilera totally blows Spears out of the water. Another thing that totally discredit her and her talent though is in 2002. when she started dressing more "raunchy" and her music video for the song "Dirrty" was a VERY sexed up video, it would wind up having a more "damaging effect" on her carrer.
But still in the end, she CAN sing and there where I think people sometimes overlook.