John Williams, probably the most famous living composer, has been nominated for 48 Oscars and has won 5. I haven't done much research in the matter, but I'm pretty certain he's the most nominated person alive. This year he could win a sixth statue for composing the original score of Steven Spielberg's Lincoln. With 12 nominations, it's no question that the movie is beloved by the Academy. In any other year, so much love for a film plus the names of Spielberg, Day-Lewis and Williams should be enough for a win. What makes me pause and think is not so much the quality of the score itself (which isn't too far away from Williams' usual work, but has a nice restrained quality), but the fact that the Golden Globes didn't give this award to Williams. As you may well know, the members of Hollywood Foreign Press Association, which gives out the Globes, love to give awards to the most famous celebrities. In the original score category, the biggest thing resembling a celebrity was Williams, so the fact that they didn't hand him the prize can be telling.
The disappointing fact is that the field of nominees doesn't offer anything really exciting as an alternative to Williams. The two best scores of the year (Beasts of the Southern Wild and The Master) weren't nominated. The most deserving of the nominees, Dario Mariannelli's work in Anna Karenina, will probably have to sit this one out since Mariannelli is a previous winner for Atonement and since Anna Karenina hasn't been very popular.
I think the fight for the win comes down to Alexandre Desplat, who doesn't by far do his best work in Argo, but has been nominated so many times and is affiliated with such a popular film that he could win and Mychael Danna whose score for Life of Pi just seems to me like the stuff the Academy likes to honor. Weirdly, the piece of music that will probably grant Pi the win is going to be "Pi's Lullaby", which is nominated in the Original Song category. The Academy, however, surely won't mind.
Will win: Life of Pi (with Argo close behind)
Should Win: Anna Karenina