For me, Christina's vocal performance is on the song "Loving Me 4 Me" on the Stripped album. It's my all-time favorite Christina. It's the one song in her catalogue where I think she really shows all the colors of her voice. Besides that, her restraint on that song is very refreshing. There have been a few songs here and there that I've truly enjoyed (Without You, Beautiful, A Song For You, Slow Down Baby, Here to Stay, Welcome, Falling In Love Again), I think that when Christina can really show some artistry on a full album like she showed on Loving Me 4 Me, that's when I'll say that her voice has gotten better. Right now, her belting seems on the same level it was back in Stripped, and aside from the clusterf*ck of genres that composes her catalog and the constant switch in sounds from album to album (which some poor souls mistake for evolution...sorry, it's lack of an identity), she hasn't shown any growth as a vocalist. I still have yet to hear a song from her that emits any sort of real, raw, emotion, save "I'm Ok."