Honestly, probably Britney.
I hate seeing Christina being written off so carelessly though. Before Bionic she was a huge single seller. Wasn't she the second biggest selling female singles artist of the decade? Beautiful and Ain't No Other Man both unfortunately stalled at #2, but that's not far off from #1. Also, Off the top of my head I think Beautiful is a platinum digital seller, and Dirrty has sold around 900k and is still selling, so clearly she has fans (or at least people who know she exists) in the digital age. Who's to say those people won't use her next single?
And I think people are underestimating what a huge platform The Voice is... How was Maroon 5 doing before they premiered MLJ? The lead single from Hands All Over failed to break the top 10 in the US and the rest of the singles struggled to float around in the top 80.
Payphone just sold 493k in a week, clearly the post-MLJ hype and premiering on one of the biggest shows in America has
some effect. I'm an Xtina fan so maybe I'm being optimistic. But she clearly has a casual fanbase of some sort, as well as a ruthless stanbase who would likely rather die before they see her underperform again. With a usable lead single, good promotion, a built-up "comeback" premiere on The Voice finale, plus the post-MLJ hype I hope she'll benefit from, I really think she's a contender for the #1 slot. But I guess we'll see.