I actually think her best Vocals are on ITZ and Blackout. That's really the Britney I love, especially because they didn't pitch the **** out of her voice.
Her best vocals are on:
Britney: not a girl not yet a woman, overprotected, slave ...
OIDIA: oops!, stronger, lucky, don't let me be the last to know, ..
Oops...is not better than Britney or Circus.
What brings down Oops... can't make you love this song, dear diarrhea, i can't get no satisfaction
Britney...That's where you take me to zzz
In the zone...flawfree but I'll throw Outrageous in there because Britney doesn't like it.
Um, Break the ice/Heaven on earth/Perfect lover, Ooh ooh baby, Gimme more and unreleased track literally have explosive diarreha on what Britney deemed her best vocals at the time what it's like to be me...
Vocally: Baby one more time, In the zone and Oops...
Quality: In the zone, Blackout and Britney. Only because circus is a summary of her 5 albums strengths.
Femme Fatale has Till the World Ends and her 2 most viewed videos, I wouldn't call it forgotten.
That's only because of the time it was released. For example had Toxic or BOMT been released today in similar circumstances it would be well over 200 million. I'm talking about that level of memorable. Till The World Ends and I Wanna Go and no where near as memorable as Britney's signature songs.
Britney sounds great and very cute in both the song and the video, ya'll will have to deal. 
