This is an especially brooding album even by The National's standards. It's sonic and spiritual cousin is Boxer IMO: somber, delicate songs only occasionally interspersed with hard-driving tracks. Songs like Green Gloves and Gospel feel like touchstones, but there is also something about Sleep Well Beast that feels very fresh. It might be the minimalist beats/electro textures running through pretty much every song; they are noticeable but never distracting, and in some cases bizarrely affecting (Nobody Else Will Be There, I'll Still Destroy You). The band is in lock-step as well; The System Only Dreams in Total Darkness is actually an outlier in how much one member of the band stands out.
Hard to pick favorites right now because the damn thing is so consistent in song quality and so well-sequenced. For example, when Matt starts singing "it all catches up to me" in Guilty Party, it feels like an absolute gut punch because of all the lyrical content that came before it, much more so than it would in isolation. I think a lot of people will see the lyrics as related to a dissolving romantic relationship, but really it could be about distance growing between any two people, or even two versions of yourself: who you are and who you wanted to be, or who you are and where you are heading. Anyway, anyone who hears this and says the band doesn't evolve is full of shit. Great, mature album.