Gotta go with [REC] (2007), I think. A more brutally efficient use of a mere 70 minute running time I don't think I've witnessed in a horror movie. Just remember to avoid the sequels. And the remakes. And the sequels of the remakes. And the remakes of the sequels. Etc.
The Wailing (2016), on the other hand, runs nearly two hours and never feels overlong or padded, even on multiple viewings. As rich and strange a horror story as I've seen in years.
Romero's Diary of the Dead (2007) should have been a late-career masterpiece, bringing timely ideas like social networking, "citizen journalism" and vlogging into play in his apocalyptic vision, but something went amiss. Whether it was hampered by budget or schedule restrictions, or by a crew not quite up to the task, I have no idea, but too much of it feels sloppy and undercooked. Nevertheless, I'm fond of it.
Otherwise, most of my favorite horror movies from this century are unlikely to appear on any major lists, and I'll be the first to admit most of them are somewhat lacking in terms of production values, to put it mildly. Some of this comes down to the fact that a good number of them are found-footage, such as Hell House LLC (2015), Evidence (2012), and the Zombie Diaries "duology" (2006/2011). Almost all of them can be watched for free (with occasional ad breaks, alas) on the streaming service Tubi, but be warned, some of the more graphic horror material on there knocks all that tepid sludge like the "Saw" franchise (and the rest of the so-called torture porn movies that followed closely in its wake) straight into a cocked hat.
(One caveat regarding Tubi: for various reasons, my movie-watching has been way down for most of this year, and I recently had to re-login to Tubi after not using it for some months, only to find that my enormous saved to-be-watched list had vanished completely. Whether this was due to some software glitch in the app or because of some severe downgrade of the service, I have no idea and I haven't yet bothered to check. But it's entirely possible that their library is no longer as extensive and eclectic as I remember it.)