March 11, 2011 at 05:00 AM EST

Britney Spears, ”Till the World Ends”
Spears’ anthemic second single, co-written by pop tart Ke$ha, more than compensates for her lethargic first outing from the upcoming Femme Fatale, ”Hold It Against Me.” Spears orders listeners to ”Keep on dancin’ till the world ends,” and thanks to the driving beat and exhilarating chorus chants, we’re happy to join this apocalyptic party. B+
—Tim Stack

Kim Kardashian, ”Jam (Turn It Up)”
The Kardashian clan has mastered the art of blank-eyed being and nothingness as a lifestyle brand (klothing line! kredit kards!), but let the buck stop — no really, stop — here. Auto-Tune can fix a wonky note, but it cannot add life to the nasal, affectless Valley-girl drawl with which Kim delivers her debut single, a song as vocally and lyrically flat as her famously badonky curves are not. Even with superproducer The-Dream (Beyoncé, Rihanna) helming, there is zero jelly in this ”Jam.” D
—Leah Greenblatt

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