As others are saying, this category is one of the most interesting on its own AND as perhaps a major signpost along the way to the best picture win.
I should be annoyed that The Big Sick is here instead of Phantom Thread, but the latter got so much more than it could have that it's hard to be bitter, and it's nice for the happy couple to have got something for its summertime hit. (Especially considering that there are some recent years the film might have won this category outright.)
It is extremely weird that the PGA/DGA winner -- and best director sure-shot -- seems to be universally thought to be running 4th here. Not, I guess, as weird as such a film not even being nominated (see: Gravity), but a sign that there's some loosening of what had for decades been universal Academy criteria.
But we don't yet have evidence that these criteria have loosened to the point that no old rules apply. And the key rules we're still dealing with are that, to win best picture, you pretty much need a screenplay or directing win. In extremely rare cases, acting + tech wins can pull you through -- in modern times, Gladiator and Chicago pulled off this unusual feat -- but I don't think any of our remaining three contenders have much hope of that (Get Out and Lady Bird because they have no tech nominations, Three Billboards because it has a couple but doesn't seem a threat to win any). Which is to say: if something besides The Shape of Water is to win best picture, unless we're going to step into unprecedented territory, it would appear the film would need to win this category.
I'm sorry to say this, because I like the film, and would rank it at worst second, but I think Lady Bird's loss at WGA was effectively an elimination round. The films were competing for "who will survive to challenge Three Billboards?", and Peele came out the winner. The only possibility this isn't so is, if what I've suggested in a few other threads turns out true: that the Cheryl Boone Isaac diversity push has finally reached the point where Academy membership now differs enough from Guild sampling that Oscar results no longer reflect these straw-poll outcomes in the way to which we've become accustomed. This is something devoutly to be wished -- it would make the Oscars an independent business once again, after a decade or more of enslavement to the damned precursors -- but, until we've seen evidence of it, it's not something you'd bet rent money on.
So, for me, it pretty much comes down to Get Out and Three Billboards, with the winner likely to be the stronger best picture challenger to Shape of Water. I truly don't know how to choose; I think many voters would like to honor both. Get Out is exactly the kind of zippy, fresh concept that the writers have often gone for as a consolation prizes (i.e., since it wasn't getting best picture) -- Eternal Sunshine, Midnight in Paris, her. And, speaking of films that would have won in other recent years...I'd say Get Out would have been an easy winner many times in the past decade-plus.
But Three Billboards is, I think, seen as a bigger effort overall -- evidenced by its nominations haul, and its multiple best film wins at Globes, SAG and BAFTA. And let's not just diminish Martin McDonagh as someone who only won prizes because he was a local boy. This is one of the world's major playwrights, whose previous nomination for In Bruges marked him as an Academy comer even as he was working his way up the directorial ladder. This is a guy who'd be as OBVIOUS a choice for screenplay in most of those years as Get Out. That he's not seen that way is partly a product of the times -- when the call is for minority and gender representation, an aging white guy winning seems a bit backward, however hip that white guy is -- and partly due to his film being divisive (which is somewhat related to those racial/gender issues, but also partly because he works in a style that isn't journalistic/realistic, which has left some audiences alienated). But I think he's going to have lots of support, and could win both this and best picture.
One thing we're maybe going to find out: which omission counts more, directing or editing? McDonagh's being left off the directing list got a lot of attention, with good reason, but Get Out's failure to secure an editing nod seems pretty important as well, because 1) Since 1980, we've actually seen more films without directing nods win best picture without director (two) than editing (one); 2) it left the film with no below-the-line nods, and I believe no film since the 30s has won best picture with that deficiency; and (3) editing seemed like the kind of category it could WIN, so missing there is no minor event. Which is to say that, for all of Get Out's clear strengths, it has a few chinks in its armor, which could be crucial in what appears to be a very close contest.