Thoughts on some of the categories:
Best Director: I actually am coming around to the idea that Miller could win this; his main competition would seem to be Inarritu, but he won last year and back-to-back wins are incredibly rare -- only twice has it happened (John Ford, 1940-1941; and Joseph L. Mankiewicz, 1949-1950).
Best Actor: Honestly, I can't see how any of these guys could beat Leo. Only Damon is representing a popular film overall, and neither he nor any of the others have any sort of narrative going for them.
Best Actress: Not that I think Rampling had much chance anyway, but boy did she self-destruct (unless it works as a covert appeal to voters who resent the implication that their nomination choices are wrong). This has been widely framed as Larson v. Ronan, and I think that's probably still true, with Larson starting to pull away. Room and Brooklyn have three of the same nominations (Picture, Actress, Adapted Screenplay), with the former also getting Director, which lends it the appearance of greater strength. I think Larson has the role advantage here; they're both tearjerker parts, but Room is a much more dramatic film.
Best Supporting Actor: Stallone seems like the favourite here, though I know some Oscarologists like Anne Thompson think that his HFPA flub hurt him significantly. If not him, I think it probably goes to Ruffalo -- well-liked actor in a well-liked film, and he's got the showiest part in the ensemble.
Best Supporting Actress: The biggest question mark still (as I posted a while ago). McAdams is the only nominee representing a BP nominee, and she has such a low-key part that I'm not sure it would be very high on the list of what Spotlight admirers would vote for. Mara is a co-lead in a film that missed BP, somewhat controversially; does Carol's support concentrate on her? Or does Vikander, another co-lead, win for having such a great year overall? And there's Winslet, who I guess would be the safe veteran choice; but she doesn't really feel like she has much career momentum for a second Oscar.
Original Screenplay: I think this is Spotlight's guaranteed win, even if it doesn't take Picture. It's the sort of film that, when people are trying to assess what about it they think is award-worthy, the writing will top the list, moreso than acting, directing, or any other aspect of the production. None of the other nominees seem like particularly strong contenders for a win (not a measure of quality, mind you).
Adapted Screenplay: Crammed with four Best Picture nominees and one near-miss that actually has more nominations than a few of the BP nominees, I don't quite know what to predict here. If you consider The Big Short one of the three notable BP contenders (along with Spotlight and The Revenant), then it's got this category to itself. If Room built more momentum I could see a case for Donoghue winning here, as the author adapting her own work for the screen.