Carey Mulligan should win, not for her Doctor Who role, but because she was darn good in An Education. But as somebody said Kristen Stewart will probably win which is cool as she was awesome in Adventureland and previously in Into the Wild however it wil be for her adequate Twillight performances that people will vote in her droves.
I don't see the three boys getting much of a look in despite Tahir Rahim being absolutely brilliant in A Prophet and Jesse Eisenberg rocking the Land films both Adventure and Zombie. Nicholas Hoult may fair well if fans of Skins vote for him, the film he's nominated for - A Single Man has yet to open over here.
Tahar Rahim put in great performance in A Prophet, so I would vote for him personally. Nick Hoult is a close second as I think he's taken some good, interesting roles since About A Boy, and developing into a better actor than many of his peers, and (so far) avoiding certain run-of-the-mill films that can stereotype
The only other one I've heard of is Jesse Eisenberg. Don't think I've seen anything that he's been in, but I recognise his face. I think I may also know Nicholas, but I have no idea who the other two are