The biggest problem in answering this is understanding what is a Bond girl?
The wiki for "Bond girl" describes them as:
A Bond girl is a character (or the actress portraying a character) who
is a love interest and/or female sidekick of James Bond in a novel,
film, or video game
But again, what does this really mean?
In the reboot of the series, Vesper is a much more important love interest than any other character. In the "original" run of films, Teresa di Vicenzo is probably the most important, as she became his wife. They're both arguably more than just "bond girls".
However, how do we define his other love interests? Is it simply somebody Bond has slept with? Or somebody he has flirted with who has enough one on one screen time with Bond to provide a genuine potential for a romantic conquest?
If we side step these potential issues for a moment, there are four potential candidates in Skyfall:
"Bond's Lover" (Tonia Sotiropoulou)
The wiki identifies her as a bond girl, as do many news outlets. She does sleep with Bond and was involved in filming for five days.
However, she is a relatively tiny character in the film.
Séverine (Bérénice Marlohe)
Like "Bond's lover", she is identified as a Bond girl by a few news outlets as well as the wiki.
However, unlike "Bond's lover", she is an intrinsic part of the plot, as the femme fatale of the story. She has plenty of screen time, was used throughout the trailer and promotions for the film and seems the best "bet" for a Bond girl.
Moneypenny and Bond flirt throughout the film, with us only learning at the end that she is Moneypenny.
She certainly satisfies the screen time requirements for a Bond girl, and does flirt with Bond throughout. However, there is no actual sexual activity between them, which is usually a cornerstone of a Bond girl.
Whilst some have argued for M as a bond girl, I struggle to reconcile this. She is absolutely integral to the plot, which satisfies that requirement, but there is zero love interest in their relationship (instead, it is based on respect).
Although this relationship is fascinating, and is one of the cornerstones of the film, if she was the Bond girl she would be totally unique in the Bond world.
How do you define a Bond girl?
By standard definitions, Séverine seems the most likely, but there are arguments to be made for all four (and there is of course no limit to the number of Bond girls in a film).