Summary: All three meet the criteria equally well, in terms of description, motivation, and means. There are two other possible candidates too, but they look pretty unlikely. Margaery and Sansa are, arguably, making similar amounts of progress towards actually achieving the prophecy, with Margaery in the lead (Dany needs to really raise her game here). There aren't any obvious clues from other prophecies. I'd personally edge Sansa over Marge as the most likely candidate, purely because it seems like more of a GRRM-esque twist; but it's too close to call.
They all have the right motivation, they all have (or, are on course to have) an army and powerful backers capable of casting down Cersei, they all are (or, are on course to be) some kind of queen (I'd say this requirement is strongly implied in "Queen you shall be... until there comes another"; but it's debatable), and they are all described as young and beautiful:
Spoiler tags contain summaries of differences on where they each are in the TV show. According to this transcript, the TV show wording is almost the same: "You'll be queen. For a time. Then comes another, younger, more beautiful, to cast you down and take all you hold dear"
- Sansa has motivation (avenging her family) and is getting an army and powerful backers (the knights of the vale, thanks to Baelish's plotting). It's easy to see how she could become a queen ("Queen of the North", if Baelish's plan to use her to take Winterfell is a success), and she's described as young and beautiful.
In the TV show: she's kinda in the same position, assuming she continues her escape. Baelish has manipulated the knights of the Vale into wanting to support Sansa, and has secured a royal decree to take them North to take Winterfell, on Sansa's behalf.
- Margaery has motivation (personal ambition and rivalry) and has an army and powerful backers (the Tyrells are among the most powerful families, with many potential allies). She's already a queen, and she's described as young and beautiful.
In the TV show: I think she's in a similar position, assuming she can talk her way out of the tricky situation with the Sparrows (seems likely)
Daenerys has motivation (considers herself rightful queen of Westeros and the Lannisters treacherous usurpers) and... kinda probably has one or more armies (new dothraki? unsullied if she can re-unite with them?) and powerful backers (Illyrio and chums, various sellsword companies, and let's call her dragons backers). She already calls herself a queen, and she's described as young and beautiful.
This is all assuming she doesn't just die of cholera on some hillside (don't do it GRRM, seriously! Yes, I know it would be a brave move and unpredictable...)
In the TV show: She's in the same situation - it all hinges on whether the giant dragon is impressive enough for her to convince these dothraki to follow her.
So they're all pretty much precisely equal on this front. Almost like the author planned it this way...
Since they're all possibilities, from here it's just future events, to which the only relevant clue is prophetic visions. I don't think any of Dany's, Melisandre's or Bran's are obviously relevant - the closest I can think of is the throne room covered in snow - but that could mean any number of things.
Those are the obvious three candidates. There are also two possible "surprise" candidates:
- Val, Mance Rayder's sister-in-law, has motivation (she's smart, uncompromising, ambitious, wants the wildlings to sweep south). Assuming Castle Black goes to chaos, she's in the best position to unite whatever's left of the wildlings, Jon Snow in whatever form he might have, and whatever's left of Stannis's faction (who consider her a "princess" and who Stannis tried to use to build alliances) - which could give her an army - and Melisandre is potentially a powerful backer. If Mance is dead, then according to the logic of Stannis's backers, she's Queen of the Wildlings, and she's described as young and beautiful.
In the TV show: she doesn't exist. Bit of an obstacle. But they've parachuted in characters at the last minute before (e.g. Ser Dontas, and it sounds like they're doing the same with the Iron Islanders next series)
- Arianne Martell, princess of Dorne, has motivation (she's already been involved in one conspiracy to stoke war), has an army (Dorne) and powerful backers (with Quentin's plot failed, it'd be natural for her to ally with - possible by marriage - Young Griff and company). This would make her arguably a queen, and she's described as young and beautiful.
In the TV show: she doesn't exist, and it seems impossible to parachute her in: we've met Doran Martell and there was nothing to suggest he has a daughter. So, seems unlikely.
I also briefly considered Asha Greyjoy, who has the motivation and could conceivably end up becoming a reaving Queen of the Iron Islands - but there's nothing to suggest she's more beautiful than Cersei, and the manoeuvring it'd take to get her in a suitable position would take another 5 books...
One more fun possibility, especially if we're not strict about the "queen" part:
Arya, wearing a beautiful young woman's face. Disguising herself as Taena Merryweather would make for an easy assassination, for example... This also gives us the fun possibility of her literally taking all that Cersei holds dear, via Jaqen/Pate style identity theft.
There's just one more thing to differentiate them: how many steps they've taken towards achieving the prophecy already.
Things Cersei holds dear:
- Her children:
- Joffrey. Marge was involved in this plot, whether she knew it or not. Sansa was indirectly involved. Too late for anyone else, probably (maybe Arianne was involved in supplying the poison somehow, with Baelish opportunistically ensuring Dorne also owed him a favour... seems needlessly complicated, even for ASOIAF!).
- Tommen. Marge is doing a good job of weaning the lad away from his mother, but hasn't finished the job.
- Myrcella. Arianne has certainly complicated things here, but hasn't finished the job. I can't think of any way Sansa or Marge could have been a factor in Tyrion sending her away to Dorne.
- Her power. The Tyrells and particularly Marge have been working on separating this from her for a long time.
- Her dignity. Cersei arguably lost this as a result of her failed scheming against Margaery.
- Jaime. Arguably, Sansa is a factor in him leaving King's Landing and re-thinking his life (via his promise). Brienne "the beauty" might be about to finish this job (in one of at least three possible ways), but I think she misses too many of the main criteria to be a candidate.
So if we take the prophecy very literally, I'd say this suggests the candidates could be narrowed to Sansa and Margaery:
- For it to be Sansa, she needs to somehow do something about Tommen and Myrcella, and then destroy Cersei's power and standing for good. Possible (though it's hard to see how or why she'd separate Cersei from Tommen and Myrcella).
- For it to be Marge, she needs to do something about Myrcella and Jaime, then continue what she's doing. Very possible: Jaime's starting to become a capable threat, and Marge seems ambitious enough that, if Myrcella became any kind of rival somehow (maybe if Tommen looked likely to die before giving her a half-Tyrell heir?), she might do whatever was necessary. Not necessarily killing: could be, arranging for her to be exiled, manipulating her into unwisely eloping, contriving some scandal, etc etc.
- The others are lagging way behind. If Arianne in any way helped procure Joffrey's poison, she could be a surprise challenger, but she'd need to get busy. For Dany, her only (indirect) influence on Cersei so far actually arguably helped her (Ned and Rob falling out over whether to send an assassin).
I'm aware of only one real-life clue - GRRM has said repeatedly that he works very hard to make his writing unpredictable. This is may be a small nudge in favour of Sansa over Dany and Marge, but it's only a small nudge:
- Cersei seems to believe that it's Marge, and her point-of-view leads readers down this path. Main characters misleading readers about how to read a prophecy is a pretty common GRRM trick.
- Daenerys is the obvious alternative candidate, and GRRM's been toying with leading readers to expect her to retake Westeros any book now for the whole series.
- There's no overt clues to prompt a reader to think of Sansa (with the possible exception of the vision of the throne room covered in snow, but there are hundreds of possible interpretations of this). In fact, Cersei has always viewed Sansa as harmless and easily manipulated. She'd therefore be a good surprise twist.